Messages is taking up space on a Mac

Where Messages keeps things

• ~/Library/Messages/Attachments — every photo, video, voice message, sticker and file in every conversation, in folders named with codes rather than names. This is almost all of the space.
• ~/Library/Messages/chat.db — the conversations themselves: the text, who sent it, when. Years of messages fit in a few hundred megabytes.

du -sh ~/Library/Messages ~/Library/Messages/Attachments 2>/dev/null

Terminal needs Full Disk Access (System Settings → Privacy & Security) to read this folder; so does Finder, which is why Shift-Command-G may show it empty until you grant it. System Settings → General → Storage shows the total under its own Messages heading.

Delete attachments without losing the conversation

The quickest way is Storage settings, which sorts them by size and knows which conversation each belongs to:

1. Open System Settings → General → Storage.
2. Click the ⓘ beside Messages.
3. Sort by size, select the photos, videos and files you no longer need, and click Delete.

The conversation stays. Where the attachment was, Messages shows a blank, or a note that it is unavailable, and the words around it are untouched. From inside Messages you can do the same for one conversation: open it, click the ⓘ at the top right, and in Photos, Links or Docs right-click an item and choose Delete.

Deleting from ~/Library/Messages/Attachments in Finder works the same way, with the same blank left behind, but the folder names tell you nothing about what is in them, so use Storage settings unless you have a reason not to.

Delete whole conversations

A group chat that has run for years is where the gigabytes usually are. Right-click it in the sidebar and choose Delete Conversation, and its attachments go with it. If the conversation matters and the attachments do not, use the step above instead.

Keep Messages

Messages → Settings → General → Keep Messages: Forever, One Year or 30 Days. Anything older than the period is deleted, attachments included, and keeps being deleted as time passes, which is the one setting that stops the folder growing without end. One Year is the sensible middle: a year of photos is kept, and the rest goes on its own.

What Messages in iCloud changes

With Messages in iCloud on (Messages → Settings → iMessage), your conversations are stored in iCloud and kept the same on every device. Two things follow:

• A deletion is a deletion everywhere. Delete an attachment, a conversation or a year of history on the Mac and it goes from your iPhone and iPad too, and from iCloud.
• The Mac still keeps a full copy. Messages in iCloud does not thin the Mac's attachments the way Optimise Mac Storage thins Photos. The folder is as large with it on as off.

So Messages in iCloud is not a way to save space on the Mac. It is a reason to look twice before deleting, because there is no other copy to fall back on.

Why it grows back

Because Messages saves every attachment it receives at full size and never removes one on its own unless Keep Messages tells it to. A busy group chat adds photos and videos every day, and a folder you clear by hand is back to the same size in months. Set Keep Messages once, and the hand-clearing stops being necessary.

Where the app comes in

RefreshYourMac measures Message Attachments as its own category, rated Review before deleting, because the conversation keeps a blank where each one was. Clean takes the category only after a confirmation that says exactly that, and moves the files to the Trash, where they stay until you empty it. A scheduled clean never touches it. The download edition has it; the Mac App Store edition does not.

The conversations themselves, in chat.db, are not a category and are never touched. For the matching folder in Mail, see Mail taking up space on a Mac; for everything else, how to free up space on a Mac.