Deleted photos stay on your Mac for 30 days, and count the whole time

You deleted four hundred photos, emptied the Trash, and the disk is exactly as full as it was. This is not a bug and it is not purgeable space. Photos, and several other Apple apps, keep their own Recently Deleted folder for 30 days, and nothing you do in Finder touches it.

Every app that has a second bin

Each is emptied separately, from inside that app, and each holds the full size of what you deleted the entire time. For a photo library that is the single most common reason a large deletion frees nothing.

Emptying them

1. In Photos, select Recently Deleted in the sidebar, then Delete All. On recent macOS this album is locked and asks for Touch ID or your password.
2. In Notes, open the Recently Deleted folder and delete permanently.
3. In Mail, the Trash mailbox per account, and check the account's setting for how long it holds.
4. Empty the Finder Trash afterwards, since some of these route through it.

Checking it worked

du -sh ~/Pictures/*.photoslibrary 2>/dev/null
df -h /System/Volumes/Data

Run those before and after. If the library barely moved, the deletion is still pending, and if the disk did not move but the library did, you are looking at purgeable space and snapshots instead, which is a different delay with a different explanation.

With iCloud Photos turned on

Deleting on the Mac deletes everywhere, and the 30 day window applies in iCloud too, so the space in your iCloud allowance also comes back only after it expires or you empty it. That timing catches people who delete photos specifically to get under an iCloud limit.

For what the library itself holds and the difference between optimised and original copies, the Photos library guide has the sizes.