Backup of iPad is way bigger than capacity

So, the kids have a 256 Gb iPad 9 wifi. The screen is broken, and I am making a backup using iTunes for PC.

The first weird thing is, in the bottom of iTunes it says "Over capacity by 56,87 Gb". It's on the long coloured bar with a graphical representation of the iPad content.

Next weird thing is, it has now been backing up for a couple of hours. It is not finished yet, but when I use file manager to calculate the size of the backup, it is currently at 303 Gb?!?

They have no iCloud subscription. Is there some magic storage that I don't know of? Will I even be able to RESTORE the backup to a 256 Gb iPad, if the backup is way bigger than that? What the freak is going on?

:) Erik J, Denmark