I have had the same situation setting up my Air 4 about a month ago. Basically, for some reason, even if you use iCloud Photo Library (which I do) and Optimised Storage is ON, it would still count the total size of your Photo library towards your backup. It is as if your iPad would download your entire iCloud Photo Library on your iPad, cross-reference it with the iCloud and then optimise it. Pretty strange if you ask me. It will also initially count your photo library towards your iPad’s storage.
My photo library is about 20 GB, so it took me about a day to get everything synchronised. Once the photo library was synched (you can check it if you open up Photos, at the very bottom of the screen), both the iPad storage and iCloud backup went back to their normal sizes.
I would not personally fiddle with turning iCloud Photo Library ON/OFF, as I remember reading on these forums that some people actually lost their stuff by doing so.
Just be patient, once your photo library is synchronised the sizes should be back to normal. ?