Mail, Messages and WhatsApp attachments eating storage: dux

Check the size first

Storage settings shows Mail and Messages as their own categories. Choose Apple menu > System Settings, then General, then Storage, and wait for the list to finish. If Messages or Mail is near the top with tens of gigabytes, this guide is worth your time. If both are a few gigabytes, look elsewhere first.

WhatsApp, Telegram and Slack do not get their own category. Their data counts toward System Data or Documents, depending on where the app keeps it.

Messages

Where attachments live

Every photo, video, voice message and file in Messages is stored in:

~/Library/Messages/Attachments

Next to it is the database with your conversations. The two belong together: the database points to files in Attachments by name. With years of group chats, the folder easily reaches 20 to 50 GB.

Trim them in Messages or Storage settings

The safe route goes through Apple’s own tools:

In Storage settings, click the Info button next to Messages.
Sort the list by size. Large videos are usually at the top.
Select what you do not need and click Delete.

You can also do it per conversation. Open a conversation in Messages, click the name at the top to open its details, and browse its photos and files. Control-click an item to delete it.

To keep this from growing again, choose Messages > Settings, click General and set Keep messages to 1 Year or 30 Days. Messages then removes older conversations, including their attachments, on its own.

If Messages in iCloud is on and space on the Mac is the only problem, the Store in iCloud recommendation in Storage settings can keep messages and attachments in iCloud, so the Mac holds only recent ones.

Mail

Two kinds of attachment storage

Mail stores attachments in two places:

Inside your mailboxes, in ~/Library/Mail. Every message Mail has downloaded, with its attachments. For iCloud, Gmail and other IMAP accounts, this is a local copy of what is on the server.
In Mail Downloads, in ~/Library/Containers/com.apple.mail/Data/Library/Mail Downloads. A copy of every attachment you opened by double clicking it.

The second one is pure duplication. The original is still in the message, so these copies can go. Mail can clean them up itself: choose Mail > Settings, click General and set Remove unedited downloads to When Mail Quits or After Message is Deleted. Copies you edited are kept, in case they hold your changes.

Shrink the mailbox side

Delete or archive large messages. Choose View > Sort By > Size to bring the heaviest messages to the top.
Remove attachments you have saved. Select a message and choose Message > Remove Attachments. The message stays. With IMAP accounts this removes the attachment from the server too, so save it first.
Erase deleted items. Choose Mailbox > Erase Deleted Items. Messages in the Trash mailbox still take space until you do.
Let Mail keep fewer attachments. The Optimize Storage recommendation in Storage settings keeps only recent attachments on the Mac when space is needed. Older ones download again when you open them.

The WhatsApp app for Mac keeps photos, videos and documents from your chats in its group container:

~/Library/Group Containers/group.net.whatsapp.WhatsApp.shared/Message/Media

On a Mac linked to an active phone, this folder can grow into tens of gigabytes, because media from busy group chats downloads automatically.

Trim it from inside WhatsApp. Open a chat’s info to see its media, and delete the large videos and files you do not need, or clear chats you no longer need. Turning off automatic media downloads in WhatsApp’s settings keeps new media from piling up. WhatsApp moves its menus around between versions, so if a setting is named differently, check WhatsApp’s own help.

As with Messages, do not delete files inside the container in Finder. WhatsApp’s database expects them, and the chat shows broken media.

Other chat apps

Slack keeps its cache in ~/Library/Application Support/Slack, mainly in the Cache and Service Worker folders. They can reach a few gigabytes and rebuild from the server, so they are fine to clear with Slack quit.
Telegram keeps downloaded media in its own group container and has a storage setting in the app that clears cached media. Your chats stay in the cloud.
Teams, Discord and other Electron apps keep caches in their folders in ~/Library/Application Support. Folders named Cache are the reclaimable part.

Measure them in Terminal

This prints the size of each location. Messages and Mail are protected, so Terminal needs Full Disk Access (System Settings > Privacy & Security > Full Disk Access) to read them.

~ % du -sh ~/Library/Messages/Attachments
 32G  /Users/you/Library/Messages/Attachments
~ % du -sh ~/Library/Containers/com.apple.mail/Data/Library/Mail\ Downloads
2.1G  …/Mail Downloads
~ % du -sh ~/Library/Group\ Containers/group.net.whatsapp.WhatsApp.shared/Message/Media
 25G  …/Message/Media

A quick order of work

Clear Mail Downloads. It only holds copies.
Delete large videos in Messages and WhatsApp that you have already saved or no longer want.
Set Keep messages and Remove unedited downloads, so the folders stay smaller.
Empty the Trash, both in Finder and with Erase Deleted Items in Mail.