Compressing a large folder to send someone
Right-click, Compress, attach — and then the email bounces, or the ZIP is barely smaller than the folder. Here is what compression can and cannot do for a large folder, and how to actually get it to the other person.
You have a folder to send — a project, a batch of photos, a set of recordings. You right-click it, choose Compress, and attach the ZIP to an email. It bounces, or the attachment is refused before you can send it, or the ZIP turns out to be almost the same size as the folder.
Each of those has a specific cause, and knowing which one you are dealing with saves a lot of retrying.
What Finder's Compress does
Right-click a folder and choose Compress "Folder Name". Finder writes a .zip next to it using standard ZIP compression. It is quick, it opens on every operating system, and for a folder of documents it is often all you need.
What it does not offer: a choice of compression level, a password, a different format, or splitting into parts.
Why the ZIP is barely smaller
Compression works by finding repetition. Text, spreadsheets, source code, uncompressed audio and many document formats are full of it, and shrink a lot.
Photographs (JPEG, HEIC), video (MP4, MOV), compressed audio (MP3, AAC), and most modern Office and PDF files are already compressed. There is nothing left to find. A folder of holiday videos will come out of any archiver within a percent or two of its original size, and no setting changes that.
So check what the folder is mostly made of before you try to make it smaller:
- Mostly text, code or raw data — compression will help, and 7z will usually beat ZIP.
- Mostly photos and video — compression is only useful for bundling into one file. The size problem has to be solved another way.
If the photos are much larger than the recipient needs, resizing them is the real saving. A 5 MB photograph resized for screen viewing is often a few hundred kilobytes.
Email is the wrong pipe for large files
Attachment limits are low, and they apply to the encoded size, which is about a third larger than the file on disk:
- Gmail rejects messages over 25 MB and offers a Google Drive link instead.
- Outlook.com and many corporate mail servers stop at around 20 MB, and some company servers are stricter still.
- The receiving server has its own limit, which you cannot see. A message your provider accepted can still bounce at the other end.
Apple Mail has a better option built in: Mail Drop. With an iCloud account, a large attachment is uploaded to iCloud and the recipient gets a download link. It handles files up to 5 GB, and the link expires after 30 days. It is the least-effort route if you use Mail.
For anything larger, or anything you want to control more tightly, a shared folder link from whichever cloud storage you already use is the normal answer.
Things that trip up the recipient
Windows users see __MACOSX folders. Finder's archives can include a hidden folder of Mac-specific metadata. It is harmless, but it looks like junk to someone on Windows. Mention it, or they will ask.
Very long paths. Deeply nested folders with long names can exceed path limits on Windows when extracted. Flattening the structure a little before compressing avoids a confusing error at the other end.
7z needs software on Windows and on older Macs. 7z compresses better than ZIP for many kinds of data, but the recipient needs something that opens it. If you do not know what they have, ZIP is the safe default.
Passwords. Finder cannot password-protect a ZIP. Traditional ZIP encryption is weak; AES-256 in a 7z archive is strong. Either way, send the password through a different channel from the file.
A sensible routine
- Look at what the folder contains. Resize photos or export smaller video first if the recipient does not need the originals.
- Compress to ZIP for anyone, or 7z if you know they can open it and the content is compressible.
- Under about 20 MB: attach it.
- Over that: Mail Drop, or a shared link.
Where Ziploom fits
Ziploom is an archive utility for Apple silicon Macs that creates ZIP and 7Z archives from files and folders, so choosing 7z for a folder of documents is a menu choice rather than a trip to the Terminal. It shows progress on large jobs, lets you decide what happens when a destination already exists, and reveals the finished archive in Finder.
In the other direction it extracts RAR, ZIP and 7Z, with optional password support, and runs safety checks for unsafe paths, symbolic links, suspicious compression ratios, oversized jobs and available disk space before anything is written. It cannot create RAR files.
It will not make a folder of videos smaller — nothing will. Everything is processed on your Mac, with no accounts, analytics, telemetry or network features.