I have three iOS devices backed up to our iCloud account. My iPhone XS (about 700 MB) my iPad mini 5 (250MB) and my wife’s iPhone 8+ - over 8 GB!!! All three devices had basically all the apps backup turned on and on my wife’s 8+ I even deleted the backup and turned off most of her apps. And it did this huge backup again. Is it because it is an older iPhone because she has a lot less apps on her iPhone than the other two devices and they all share the same Apple photo library with optimize storage turned on.