1. See what Messages is storing
Open System Settings → General → Storage. Apple’s storage guidance lists Messages among the categories with their own management options; its information button opens the attachments Messages holds, sorted by size, and lets you select and delete the ones you no longer want.
From the file side, a disk scanner shows the Messages folder inside your Library, where the app keeps its database and an Attachments folder. Treat it as review-only. The safe way to shrink it is through the app, because the database tracks every file and a deletion in Finder leaves conversations pointing at attachments that no longer exist.
2. Delete attachments inside a conversation
Open the conversation and show its details. Apple’s guide describes scrolling to the categories such as Photos, Shift-clicking to select several items, then Control-clicking and choosing Delete. This removes the attachment from the conversation while keeping the messages around it.
Deleted messages and attachments stay in Recently Deleted for up to 30 days, so the space does not return immediately. That window is the safety net; it also means a large cleanup shows up in Storage settings only after it expires.
3. Set old conversations to expire
Choose Messages → Settings → General and open the Keep messages menu. Apple’s warning is worth quoting in spirit: pick anything other than Forever and conversations, including all attachments, are removed automatically once the period passes. For a Mac that only mirrors an iPhone, one year is a reasonable setting; for a Mac that holds the only copy of something, keep Forever and delete by hand.
This is the setting that stops the folder growing back. Deleting attachments once is a cleanup; a retention period is maintenance.
4. Understand what Messages in iCloud changes
With Messages in iCloud turned on, Apple’s guide states that deleting a message or conversation on your Mac deletes it from every device where the feature is on. Deleted items can be recovered for 30 days and are permanently removed from iCloud after 40 days. Delete with that in mind; the Mac is not a spare copy of your iPhone’s history.
If you want the Mac to hold less while the iPhone keeps everything, deleting is the wrong tool. Leave the conversations in place and rely on the retention setting only where you accept the loss on all devices.
5. Check the result
Compare the Messages figure in Storage settings after the Recently Deleted window, or after clearing Recently Deleted from the conversation list. Rescan with a disk tool to confirm the Attachments folder actually shrank; if it did not, the items are still within their 30 days.
When Messages was not the largest category after all, the guides on System Data, Mail and large files cover the usual next places. ClearDisk’s free scan shows the Messages and Mail folders with allocated sizes but does not offer them for removal, because their apps own the data.
References & further reading
These guides combine the linked official guidance with ClearDisk’s documented behavior. Your files and app settings determine the right next step.