In short
- Optimize Mac Storage keeps originals in iCloud and smaller copies on your Mac.
- Deleted photos stay in Recently Deleted for 30 days unless you remove them sooner.
- Keep messages controls how long conversations and their attachments stay.
- With iCloud on, deleting on your Mac deletes on your other devices too. Never delete the Photos Library file itself.
Photos: keep originals in iCloud
If you use iCloud Photos, this is the option that frees the most without deleting anything. In Photos, choose Settings, click iCloud, turn on iCloud Photos and select Optimize Mac Storage. Apple says your Mac then keeps smaller versions when space is limited, and the full-size originals stay in iCloud (Apple's steps).
It only helps if your iCloud plan has room for your whole library. Originals also have to finish uploading before the space can come back.
Photos: delete what you don't want
Deleting a photo doesn't free its space straight away. It moves to the Recently Deleted album, and Apple says items there are permanently deleted after 30 days. You can remove them from that album sooner (Apple's steps).
With iCloud Photos on, a deletion happens everywhere. Apple says deleting a photo on one device removes it from every device using iCloud Photos.
Long videos are usually the quickest win, because one can take as much space as thousands of photos.
Don't delete the library file
Your photos live in a single file in the Pictures folder, called Photos Library. It can be the largest item on your Mac, but deleting it deletes your library. Make it smaller from inside the Photos app.
Messages: keep them for less time
Messages keeps conversations and attachments forever unless you change it. In Messages, choose Settings, click General, and use the Keep messages menu. Apple says any option other than Forever removes older conversations automatically, with all their attachments (Apple's page).
That is permanent housekeeping, so check for anything you want to keep before choosing a shorter period.
Messages: remove large attachments
System Settings → General → Storage lists recommendations and what each category is using, and it is the first place to look. You can also delete conversations in Messages, and they stay in Recently Deleted for up to 30 days.
With Messages in iCloud on, deleting a conversation on your Mac removes it from your other devices too (Apple's page).
How StorageJanitor handles these
It marks your Photos library and Messages as Keep, so it never removes them, and it says how to shrink each one yourself. It shows how big they are next to everything else on your disk, so you can decide whether they are the place to start.
Questions people ask
Is it safe to delete the Messages attachments folder in Finder?
We wouldn't. Messages keeps track of those files, so use the app or System Settings, which keep it consistent.
Why is Photos still large after I deleted photos?
Deleted items stay in Recently Deleted for 30 days unless you remove them from that album sooner, and they use space until then.
Does Optimize Mac Storage delete my photos?
No. Apple says full-size originals stay in iCloud and your Mac keeps smaller versions. You need enough iCloud storage for the whole library.
Will these steps free space immediately?
Not always. Recently Deleted holds items back, and macOS can take time to hand space back after iCloud finishes uploading.