That's great info, it's hard to come across stuff like this. I've been wondering what larger libraries use for a while.
On the 7 Plus it had slowly grown to use 60GB, but part of that definitely was a combination of ~1 year worth of photo taking and a lot of empty storage space, so it likely never had to purge any older photos.
For comparison, my 256GB iPad that’s been in use for 3 months has 100GB storage used in total, and 58GB used by photos. I frequently use the iPad to browse and edit photos, make albums, etc.