{"id":1969,"date":"2026-10-21T15:13:00","date_gmt":"2026-10-21T19:13:00","guid":{"rendered":"https:\/\/www.xconvert.com\/blog\/?p=1969"},"modified":"2026-08-31T23:06:01","modified_gmt":"2026-09-01T03:06:01","slug":"why-zipping-doesnt-shrink-files","status":"publish","type":"post","link":"https:\/\/www.xconvert.com\/blog\/why-zipping-doesnt-shrink-files","title":{"rendered":"Why ZIP Won&#8217;t Shrink Your JPGs, MP4s or PDFs (and What Does)"},"content":{"rendered":"\n<p class=\"wp-block-paragraph\">You have a 480 MB folder of holiday photos and a 900 MB screen recording, the upload form caps at 500 MB, so you right-click and zip it. Windows churns for two minutes and hands back a file that is 478 MB \u2014 or, for the video on its own, very slightly <em>larger<\/em> than the original. This is not a broken zipper, a wrong setting, or a slow computer. It is exactly what the ZIP specification and the compression algorithm inside it are designed to do, and the reason is worth understanding once: it tells you precisely which files ZIP will help with and what to do about the ones it won\u2019t.<\/p>\n\n\n\n<blockquote class=\"wp-block-quote is-layout-flow wp-block-quote-is-layout-flow\"><p><strong>Quick answer:<\/strong> ZIP compresses with <strong>DEFLATE<\/strong>, which shrinks data by finding <strong>repetition<\/strong> and replacing it with back-references. JPG, HEIC, PNG, MP4, MOV, MP3, AAC, GIF and PDF have all already had their repetition squeezed out by their own codecs, so DEFLATE finds nothing left to remove \u2014 and the ZIP container adds roughly <strong>76 bytes of structural overhead per file<\/strong> on top, which is why a zipped video can come out fractionally bigger. RFC 1951 states the principle outright: \u201cA simple counting argument shows that no lossless compression algorithm can compress every possible input data set.\u201d ZIP\u2019s real value on media is <strong>bundling<\/strong>, not shrinking. To make media smaller you have to re-encode the media itself.<\/p><\/blockquote>\n\n\n\n<h2 class=\"wp-block-heading\">Jump to a section<\/h2>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"#deflate\">What ZIP actually does to your bytes<\/a><\/li><li><a href=\"#already\">Why already-compressed files have nothing left to give<\/a><\/li><li><a href=\"#bigger\">Zipping can make a file bigger \u2014 here\u2019s the arithmetic<\/a><\/li><li><a href=\"#routing\">What actually shrinks it: a routing table<\/a><\/li><li><a href=\"#good-for\">What ZIP is genuinely good at<\/a><\/li><li><a href=\"#tool\">Make a ZIP on xconvert<\/a><\/li><li><a href=\"#faq\">FAQ<\/a><\/li><\/ul>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"deflate\">What ZIP actually does to your bytes<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">A ZIP file does two separate jobs, and people conflate them: it <strong>bundles<\/strong> many files into one container, and it <strong>compresses<\/strong> each of them. Only the second job affects file size, and it is done by an algorithm called <strong>DEFLATE<\/strong> \u2014 compression method 8 in the ZIP specification, which PKWARE\u2019s own format document calls \u201cthe method used by default by most ZIP compatible application programs.\u201d<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">DEFLATE is defined in RFC 1951, and its mechanism is simple to describe. It \u201ccompresses data using a combination of the LZ77 algorithm and Huffman coding\u201d. In practice that means the output is made of two kinds of thing: \u201cliteral bytes (of strings that have not been detected as duplicated within the previous 32K input bytes), and pointers to duplicated strings\u201d.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">So DEFLATE\u2019s entire saving comes from <strong>finding sequences of bytes it has seen before within the last 32 KB and replacing them with a short reference<\/strong>. That\u2019s it. Where a file is full of repetition \u2014 English prose, CSV columns, log lines, XML tags, source code, the long runs of identical pixels in a BMP \u2014 that works spectacularly. RFC 1951 gives its own figures: \u201cEnglish text usually compresses by a factor of 2.5 to 3; executable files usually compress somewhat less; graphical data such as raster images may compress much more.\u201d<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Where there is no repetition left, it does nothing.<\/p>\n\n\n\n<figure class=\"wp-block-image size-large\"><img loading=\"lazy\" decoding=\"async\" width=\"1600\" height=\"477\" src=\"https:\/\/www.xconvert.com\/blog\/wp-content\/uploads\/2026\/08\/zip-numbers.png\" alt=\"Three headline numbers: DEFLATE's worst-case size increase of 0.015 percent, ZIP's 76 bytes of structural overhead per file, and the 2.5 to 3 times compression RFC 1951 reports for English text\" class=\"wp-image-1968\" srcset=\"https:\/\/www.xconvert.com\/blog\/wp-content\/uploads\/2026\/08\/zip-numbers.png 1600w, https:\/\/www.xconvert.com\/blog\/wp-content\/uploads\/2026\/08\/zip-numbers-300x89.png 300w, https:\/\/www.xconvert.com\/blog\/wp-content\/uploads\/2026\/08\/zip-numbers-1024x305.png 1024w, https:\/\/www.xconvert.com\/blog\/wp-content\/uploads\/2026\/08\/zip-numbers-768x229.png 768w, https:\/\/www.xconvert.com\/blog\/wp-content\/uploads\/2026\/08\/zip-numbers-1536x458.png 1536w\" sizes=\"auto, (max-width: 1600px) 100vw, 1600px\" \/><\/figure>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"already\">Why already-compressed files have nothing left to give<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Every format below already ran its own compression pass \u2014 usually a much cleverer one, tailored to its data \u2014 before the file ever reached your disk:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>JPG \/ JPEG \/ HEIC<\/strong> \u2014 the image is transformed, quantised and then entropy-coded. The output is close to noise as far as a byte-matcher is concerned.<\/li><li><strong>PNG<\/strong> \u2014 this one surprises people: the PNG specification defines exactly one compression method, and it <em>is<\/em> DEFLATE. \u201cPNG compression method 0 is deflate compression with a sliding window\u2026 of at most 32768 bytes,\u201d and \u201cAll conforming PNG images shall be compressed with this scheme.\u201d Zipping a PNG is asking DEFLATE to compress its own output.<\/li><li><strong>GIF<\/strong> \u2014 LZW-compressed, a close relative of the same idea.<\/li><li><strong>MP4 \/ MOV \/ MKV<\/strong> \u2014 H.264, HEVC or AV1 inside, which remove redundancy <em>between<\/em> frames as well as within them. Far more aggressive than anything a general-purpose zipper can do.<\/li><li><strong>MP3 \/ AAC \/ M4A \/ Opus<\/strong> \u2014 perceptual codecs that permanently discarded the parts of the signal you can\u2019t hear, then entropy-coded what was left.<\/li><li><strong>PDF<\/strong> \u2014 content streams and embedded images are typically already Flate-compressed (the same DEFLATE), which is why the <a href=\"https:\/\/www.xconvert.com\/compress-pdf\">PDF compressor\u2019s Ghostscript presets<\/a> work by <em>resampling images<\/em>, not by squeezing bytes.<\/li><li><strong>DOCX \/ XLSX \/ PPTX \/ EPUB \/ JAR \/ APK<\/strong> \u2014 these are not merely compressed, they are <strong>already ZIP archives<\/strong>. Microsoft\u2019s own Open XML documentation tells you to \u201crename the extension on the minimum document from .docx to .zip\u201d to look inside; the parts of the document are files in a ZIP package. Zipping a <code>.docx<\/code> wraps one ZIP inside another.<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">That\u2019s the whole explanation. There is no setting, no \u201cmaximum compression\u201d level and no better zip utility that changes it \u2014 the redundancy DEFLATE hunts for was already spent by the codec that made the file.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"bigger\">Zipping can make a file bigger \u2014 here\u2019s the arithmetic<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Two separate effects push the other way.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>DEFLATE itself has a floor.<\/strong> RFC 1951 is explicit: \u201cA simple counting argument shows that no lossless compression algorithm can compress every possible input data set. For the format defined here, the worst case expansion is 5 bytes per 32K-byte block, i.e., a size increase of 0.015% for large data sets.\u201d Tiny, but not zero \u2014 and it applies precisely to the incompressible data you\u2019re most likely to be zipping.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><strong>The ZIP container has fixed per-file overhead.<\/strong> From the .ZIP File Format Specification, every file inside an archive is described twice:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li>a <strong>local file header<\/strong> \u2014 30 fixed bytes, plus the filename and any extra field;<\/li><li>a <strong>central directory file header<\/strong> \u2014 46 fixed bytes, plus the filename again, plus extra field and comment.<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">Then the archive as a whole carries a 22-byte <strong>end of central directory record<\/strong>. So each file costs at least <strong>76 bytes of structure plus two copies of its name<\/strong>, before a single byte of content. Zip 500 photos and you\u2019ve added roughly 38 KB of bookkeeping \u2014 invisible against 2 GB of JPEGs, but it\u2019s real, and on a single already-compressed file it is the reason the ZIP can be marginally larger than what went in.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Good zip utilities notice this and fall back to compression <strong>method 0 \u2014 \u201cthe file is stored (no compression)\u201d<\/strong> \u2014 for data that DEFLATE couldn\u2019t improve. That\u2019s why a zipped MP4 usually comes out at \u201csame size plus a few dozen bytes\u201d rather than measurably larger. The archive isn\u2019t failing; it correctly decided there was nothing to gain.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"routing\">What actually shrinks it: a routing table<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">If the goal is fewer bytes rather than fewer files, you have to re-compress the content in its own format. Use this to route:<\/p>\n\n\n\n<figure class=\"wp-block-table\"><table><thead><tr><th>What you have<\/th><th>Why ZIP can\u2019t help<\/th><th>What actually makes it smaller<\/th><\/tr><\/thead><tbody><tr><td>JPG \/ JPEG \/ HEIC photos<\/td><td>Already DCT-transformed and entropy-coded<\/td><td>Re-encode at lower quality or fewer pixels \u2014 <a href=\"https:\/\/www.xconvert.com\/compress-jpeg\">JPEG compressor<\/a><\/td><\/tr><tr><td>PNG screenshots and graphics<\/td><td>PNG <em>is<\/em> DEFLATE internally<\/td><td>Reduce the colour palette or resize \u2014 <a href=\"https:\/\/www.xconvert.com\/compress-png\">PNG compressor<\/a><\/td><\/tr><tr><td>MP4 \/ MOV \/ MKV video<\/td><td>Inter-frame codec has removed far more than DEFLATE could<\/td><td>Lower the bitrate or resolution \u2014 <a href=\"https:\/\/www.xconvert.com\/compress-mp4\">MP4 compressor<\/a><\/td><\/tr><tr><td>MP3 \/ AAC \/ M4A audio<\/td><td>Perceptual codec already discarded inaudible data<\/td><td>Lower the bitrate, or go mono for speech \u2014 <a href=\"https:\/\/www.xconvert.com\/audio-compressor\">audio compressor<\/a><\/td><\/tr><tr><td>Animated GIF<\/td><td>LZW-compressed already<\/td><td>Fewer colours, fewer frames, smaller frame \u2014 the <a href=\"https:\/\/www.xconvert.com\/compress-gif\">GIF compressor\u2019s<\/a> <strong>Max file size<\/strong> mode<\/td><\/tr><tr><td>PDF<\/td><td>Streams usually already Flate-compressed<\/td><td>Downsample the images inside it \u2014 <a href=\"https:\/\/www.xconvert.com\/compress-pdf\">PDF compressor<\/a><\/td><\/tr><tr><td>DOCX \/ XLSX \/ PPTX<\/td><td>The file is already a ZIP archive<\/td><td>Shrink the images <em>before<\/em> you place them in the document<\/td><\/tr><tr><td>WAV \/ AIFF \/ BMP \/ uncompressed TIFF<\/td><td>Nothing has compressed it yet<\/td><td>ZIP genuinely helps \u2014 or convert to FLAC \/ PNG for a much bigger win<\/td><\/tr><tr><td>Text, CSV, JSON, logs, source code<\/td><td>Nothing has compressed it yet<\/td><td>ZIP is exactly the right tool; expect a large reduction<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n<p class=\"wp-block-paragraph\">The pattern is easy to remember: <strong>ZIP helps when nothing has compressed the file yet.<\/strong> Anything that already ends in a media extension has been compressed once, and the only way to make it smaller is to compress it again <em>as that media<\/em>, accepting the quality cost that comes with it.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"good-for\">What ZIP is genuinely good at<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\">Dropping the size expectation doesn\u2019t make ZIP useless \u2014 it makes it useful for the right reasons:<\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><strong>One attachment instead of forty.<\/strong> Email clients, ticketing systems and upload forms almost all handle a single file better than a pile of them.<\/li><li><strong>Preserving folder structure.<\/strong> A ZIP carries the directory tree; forty loose attachments don\u2019t.<\/li><li><strong>Universal extraction.<\/strong> Windows and macOS both open ZIP with a double-click, no software to install \u2014 unlike RAR or 7z.<\/li><li><strong>Real compression on the right content.<\/strong> Log bundles, database dumps, CSV exports, code and design source files often shrink dramatically, exactly as RFC 1951\u2019s 2.5\u20133\u00d7 figure for text suggests.<\/li><li><strong>A single integrity-checked container.<\/strong> Every entry carries a CRC-32, so a corrupted transfer is detected rather than silently accepted.<\/li><\/ul>\n\n\n\n<p class=\"wp-block-paragraph\">If you need to get <em>under<\/em> a hard size limit, the sequence that actually works is: <strong>compress the media first, then zip the results.<\/strong> Zipping first and hoping is the step that wastes the afternoon.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"tool\">Make a ZIP on xconvert<\/h2>\n\n\n\n<ol class=\"wp-block-list\"><li>Open <a href=\"https:\/\/www.xconvert.com\/archive-file-to-zip\">xconvert.com\/archive-file-to-zip<\/a> and click <strong>Upload<\/strong> to add your files \u2014 or drag them onto the page.<\/li><li>Add every file you want in the archive; they\u2019ll all go into one <code>.zip<\/code>.<\/li><li>Click <strong>Archive<\/strong> and download the <code>.zip<\/code>.<\/li><li>Going the other way? The <a href=\"https:\/\/www.xconvert.com\/extract-zip\">ZIP extractor<\/a> unpacks an archive without installing anything \u2014 it uploads the file, expands it on our servers and hands the contents back.<\/li><\/ol>\n\n\n\n<p class=\"wp-block-paragraph\">Because the output format is fixed to ZIP, there is nothing else to configure \u2014 and, as above, no setting that would make an archive of already-compressed media any smaller.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Your files upload over an encrypted connection, are processed on our servers, and are deleted automatically a few hours later.<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">For the step-by-step version on Windows and Mac, see <a href=\"https:\/\/www.xconvert.com\/blog\/make-a-zip-file-online\/\">how to make a ZIP file online<\/a>; for whether a different archive format would do better on your content, <a href=\"https:\/\/www.xconvert.com\/blog\/zip-vs-rar-vs-7z\/\">ZIP vs RAR vs 7z<\/a>.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\" id=\"faq\">FAQ<\/h2>\n\n\n\n<h5 class=\"wp-block-heading\">Why doesn\u2019t zipping make my files smaller?<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Because ZIP\u2019s DEFLATE algorithm only saves space by replacing <strong>repeated<\/strong> byte sequences, and JPG, PNG, MP4, MP3, GIF and PDF files have already had their repetition removed by their own codecs. There is nothing left for DEFLATE to find. ZIP still bundles them into one container, which is genuinely useful \u2014 it just won\u2019t reduce the byte count.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">Why is my ZIP file bigger than the original?<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Two reasons, both documented. DEFLATE has a worst-case expansion of \u201c5 bytes per 32K-byte block, i.e., a size increase of 0.015%\u201d on data it can\u2019t compress, and the ZIP container itself adds a 30-byte local header plus a 46-byte central-directory entry per file \u2014 plus the filename twice and a 22-byte end-of-archive record. On a single already-compressed file that overhead is the whole story.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">Does using \u201cmaximum compression\u201d or a better zip program help?<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Barely. Higher DEFLATE levels make the algorithm search harder for repetition, which costs time; if the repetition isn\u2019t there, a longer search finds nothing. 7z and RAR use stronger algorithms and will beat ZIP on compressible content, but they face the same wall on media \u2014 see <a href=\"https:\/\/www.xconvert.com\/blog\/zip-vs-rar-vs-7z\/\">ZIP vs RAR vs 7z<\/a> for where the difference is real.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">Why does zipping a DOCX or XLSX gain nothing at all?<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Because those files are already ZIP archives. Microsoft\u2019s Open XML documentation says so directly \u2014 rename a <code>.docx<\/code> to <code>.zip<\/code> and you can open it and see the document\u2019s parts inside. Zipping one just nests an archive inside another archive and adds a fresh set of headers.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">What kinds of files DO compress well in a ZIP?<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Anything that hasn\u2019t been compressed yet: plain text, CSV, JSON, XML, log files, source code, database dumps, uncompressed WAV or AIFF audio, BMP images and uncompressed TIFF scans. RFC 1951\u2019s own guidance is that \u201cEnglish text usually compresses by a factor of 2.5 to 3\u201d \u2014 that\u2019s a 60\u201367% reduction, and structured data with repetitive fields often does better still.<\/p>\n\n\n\n<h5 class=\"wp-block-heading\">How do I actually get a folder of photos and videos under an upload limit?<\/h5>\n\n\n\n<p class=\"wp-block-paragraph\">Compress the media itself, then bundle. Run the photos through the <a href=\"https:\/\/www.xconvert.com\/compress-jpeg\">JPEG compressor<\/a> and the video through the <a href=\"https:\/\/www.xconvert.com\/compress-mp4\">MP4 compressor<\/a> \u2014 both let you aim at a target size \u2014 and only then put the smaller results into a ZIP for a single tidy upload. Doing it in the other order accomplishes nothing.<\/p>\n\n\n\n<h2 class=\"wp-block-heading\">Sources<\/h2>\n\n\n\n<p class=\"wp-block-paragraph\"><em>Last verified 2026-08-31.<\/em><\/p>\n\n\n\n<ul class=\"wp-block-list\"><li><a href=\"https:\/\/www.rfc-editor.org\/rfc\/rfc1951\">RFC 1951 \u2014 DEFLATE Compressed Data Format Specification version 1.3<\/a> \u2014 DEFLATE \u201ccompresses data using a combination of the LZ77 algorithm and Huffman coding\u201d; literals and \u201cpointers to duplicated strings\u201d within the previous 32K bytes; \u201cno lossless compression algorithm can compress every possible input data set\u201d; worst-case expansion of 5 bytes per 32K block (0.015%); English text compressing by a factor of 2.5 to 3.<\/li><li><a href=\"https:\/\/pkware.cachefly.net\/webdocs\/casestudies\/APPNOTE.TXT\">PKWARE \u2014 .ZIP File Format Specification, APPNOTE.TXT v6.3.10<\/a> \u2014 compression method 0 \u201cthe file is stored (no compression)\u201d and method 8 Deflate as the default; the 30-byte local file header, 46-byte central directory file header and 22-byte end of central directory record.<\/li><li><a href=\"https:\/\/www.w3.org\/TR\/png-3\/\">W3C \/ ISO \u2014 Portable Network Graphics (PNG) Specification (Third Edition)<\/a> \u2014 \u201cPNG compression method 0 is deflate compression with a sliding window\u2026 of at most 32768 bytes\u201d and \u201cAll conforming PNG images shall be compressed with this scheme.\u201d<\/li><li><a href=\"https:\/\/learn.microsoft.com\/en-us\/office\/open-xml\/word\/structure-of-a-wordprocessingml-document\">Microsoft Learn \u2014 Structure of a WordprocessingML document<\/a> \u2014 a <code>.docx<\/code> is a ZIP package; rename the extension to <code>.zip<\/code> to inspect the parts inside.<\/li><\/ul>\n","protected":false},"excerpt":{"rendered":"<p>Zipping photos and video barely helps: DEFLATE only removes repetition, and JPG, MP4 and PDF are already compressed. Here&#8217;s what actually shrinks them.<\/p>\n","protected":false},"author":4,"featured_media":1967,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[5,14],"tags":[],"class_list":["post-1969","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-how-to-guides","category-tools"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.9 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Why ZIP Won&#039;t Shrink Your JPGs, MP4s or PDFs (and What Does)<\/title>\n<meta name=\"description\" content=\"Zipping photos and video barely helps: DEFLATE only removes repetition, and JPG, MP4 and PDF are already compressed. 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