We’ve had this bug on several Macs so far, but the symptoms are, the Mac refuses to synchronize up to iCloud, it says you need gobs and gobs of space, but when you look at the photos, there are not nearly that many .
Even if you completely deleted everything out of Photos and get rid of all the Recently deleted, you still have a problem.
This seems to be a bug in Photos, if you look at Pictures/Photo Library
 you will see a massive file, I’ve seen one that takes up 40GB, but there is literally nothing in Photos.
The fix is:
- Backup all your photos somewhere by exporting them.
- Close Photos
- Go to Finder and manually delete
Pictures/Photo Library
- Restart Photos and it will say it can’t find the
Photo Library
 and click onCreate New
- Exit Photos (because it is now confused about this new library being the special
System Library
) - Restart Photos and then go to
Preferences/iCloud Photo Library
 and you can now restart and the sync begins again.