Why zipping photos and videos barely makes them smaller

You select forty photos, choose Compress, and the zip that appears is a rounding error smaller than the folder. That is the expected result, not a failure of the Mac. Photos and videos are already compressed files, and zip cannot compress something twice.

Understanding why takes two minutes, and it points at the thing that will actually get your folder under an upload limit.

What zip is actually doing

Zip compression looks for repetition. A text file that says “invoice” three hundred times can store the word once and refer back to it, which is why a 5 MB log file can collapse to 300 KB. The same trick works on spreadsheets, code, uncompressed audio and plain documents, because all of them contain long runs of predictable bytes.

A JPEG has already had that treatment applied, by a codec designed specifically for images. So has an H.264 video, an MP3, a HEIC photo and a PDF that contains scanned pages. By the time the file lands on your disk, the obvious repetition is gone. Zip opens it, finds nothing left to exploit, and stores the bytes as they are, plus a small amount of bookkeeping.

That is why a zip of photos sometimes comes out marginally larger than the originals. Nothing has gone wrong. You paid a few kilobytes for the directory listing and got no compression back.

What zip still buys you

It is worth being clear that the archive is not pointless:

1. One file instead of two hundred. Upload portals and email clients handle a single attachment far more predictably than a large selection.
2. A checksum per entry. Zip records a CRC for each file, so an archive that survives extraction has almost certainly arrived byte for byte.
3. Folder structure preserved. Subfolders, ordering and names survive the trip, including on a Windows machine at the other end.
4. Fewer of the stray files that the Finder scatters, since the archive travels as one item.

So keep zipping when the goal is tidiness or integrity. Just stop expecting it to solve a size problem.

What actually makes a video smaller

Video is usually the real weight in a folder. The only way to make an H.264 or HEVC file meaningfully smaller is to re-encode it, which means deciding what quality you are willing to lose. There are three dials:

• Resolution. Going from 4K to 1080p removes three quarters of the pixels and is invisible on a phone screen.
• Bitrate. The biggest lever, and the one that shows up as blockiness in fast motion if you push it too far.
• Frame rate. Rarely worth touching for normal footage, and the first thing to cut for a screen recording of mostly static content.

If you have a hard ceiling, like a portal that stops at 100 MB, working backwards from the target is far more reliable than guessing at a quality slider. How to shrink a video to a specific file size walks through that arithmetic. Manatee does the same thing by naming the number: you say 90 MB, it finds the quality that fits, and it writes a new file next to the original rather than replacing it.

One caution worth repeating: each re-encode is a fresh generation of loss. Compressing an already compressed export makes the result worse than compressing the original once, which is the subject of why compressing a video twice makes it look worse. Go back to the master file when you still have it.

What actually makes photos smaller

For a folder of images, the question is what format they are in now.

If they are already JPEGs at a sensible quality, there is not much to reclaim without visible damage. Converting them to HEIC typically halves the storage for the same apparent quality, which matters for a folder you are archiving, though it costs you compatibility with older software. Converting photos to HEIC to save space covers the tradeoff.

If they are PNGs, and especially if they are screenshots, you are carrying a lossless format for content that does not need one. A 6 MB Retina screenshot becomes a few hundred kilobytes as a JPEG with no perceptible change on screen. Converting screenshots from PNG to JPEG in bulk is the batch version.

If they are camera raw files, CR2, NEF, ARW or DNG, the zip really is useless and the files really are enormous, because raw is close to a direct dump of the sensor. Converting to JPEG for sharing and keeping the raw files at home is the standard workflow, described in converting raw photos to JPEG.

When a zip does help a lot

The rule is simple: zip helps when the contents are not already compressed. You will see real savings on folders of text, CSV exports, logs, JSON, XML, WAV or AIFF audio, uncompressed TIFF scans, and older Office documents in the .doc and .xls formats. A folder of WAV files can drop by a third or more, though converting them to FLAC does considerably better and stays lossless.

Modern Office formats, DOCX and XLSX and PPTX, are themselves zip archives with a different extension. Zipping them again gains nothing, which is also why a damaged one behaves so much like a corrupted zip archive.

A routine that works for sending a folder

1. Check the size of the largest few items first. One video is usually responsible for most of the total.
2. Shrink the video to a named target, or convert it to H.264 at 1080p if the recipient’s device is unknown.
3. Convert PNG screenshots and any raw photos to JPEG.
4. Zip the result, for tidiness and for the per-file integrity check.
5. Compare against the recipient’s real limit, which is often lower than advertised once encoding overhead is counted. Email attachment size limits by provider has the numbers.

Done in that order, the zip is the last step and the smallest part of the job. Done the other way around, you spend ten minutes waiting for an archive that saves you nothing.

Questions

Does a different archive format compress photos better? No, and for the same reason. The gains between archive formats show up on compressible data. On a folder of JPEGs or MP4s, every archiver lands within a percent or two of the original size.

Does zipping reduce photo quality? Never. Zip is lossless by definition: what comes out is byte for byte what went in. That is exactly why it cannot make an already compressed photo smaller.

Why is my zip slightly bigger than the folder? Each entry carries a header, a name and a checksum, and incompressible data gets stored as is. On hundreds of already compressed files, that overhead is larger than the savings.

Can I strip metadata to save space? Metadata is usually a few kilobytes per photo, so it is not a size strategy, but it is a privacy one. How to check a file’s metadata on a Mac shows what is in there before you send it.