Are you using iCloud to synchronize your Photos? If yes, are you using "Optimize Storage?" If you optimize storage, then each copy you save may not have full sized images-- those may be loaded to your computer only when you need full size to edit or print, etc. So you may be backing up only smaller sized images, not the real things. In that case, you should turn off Optimize and backup after the full sized images are restored. (When that happens is pretty iffy.) Sorry to start with this if you're not using iCloud. But if you are, you might need to ask about that iffy part.
If you're not using iCloud, then the difference between old backups and new backups will be twofold: one difference will be any pictures you've deleted. That's a point with backups, after all, is in case you mess up and delete something you're sorry about later. So after awhile, when you're pretty sure you won't regret it, you can delete the old libraries.
The second difference is the MacOS that was used with the old libraries. For each new version of MacOS, the Photos database changes, so that a library from new Ventura can't be read by a Mac using older Monterey. So, if you're sure you don't have any computers around that you want to keep on older OSs, then you can delete the old libraries.
Frankly, I've gotten burned a couple of times from crashed hard drives, so I keep everything-- and I have way to many old hard drives.