Photos using a space of ~2GB but there is nothing in the photos app

Solution 1:

Open Terminal App and issue the following command:

du -h ~/Pictures/Photos\ Library.photoslibrary

That should list what's taking up the space. There's a whole tree of folders inside the "Photos Library.photoslibrary" directory. All sorts of metadata, databases, etc.

If you have nothing to retain, you can just delete the "Photos Library.photoslibrary" directory and if you relaunch the Photos app it will recreate it in a default state. There must be left over metadata after you deleted all the photos.

PS: You may need to restart the mac to see the effect in the storage app.