I'm sorry. I've got no ideas then. It's probably left over data in the photos database. Something the system isn't dong as good a job of cleaning up as it could.
BTW, the amount of usage on the iPad 3 sounds about right for that few photos. There's always a minimum amount taken up, photos or no.
Not that it's likely to help, but you could try a restart or reset and see if that kicks things off. Wait a while after , to give the system time to do it's thing.
Another thing you could try is to take a few photos on the iPad Mini, delete them, delete them again in the recently deleted album, then restart. Again, wait a bit.
Good luck.
----
B]Restart:[/B] Hold down the power button until the power off slider appears. Slide it off. When the iPad finishes shutting down, hold down the power button again until the Apple logo appears. Apple recommends you restart the iPad at least once a month.
and then a reset if that does not work.
Reset: Hold down the power and home buttons until the Apple logo appears. The iPad will reboot. This usually takes a bit longer than a restart.