Size of folders in files app

How can I find out the size used by each folder in the files app?

Apparently when I hit "Info" on a folder it always displays "--" but not the size used by the folder. :(

Why do I care? Files uses almost 9 GB (as reported in Settings) and I cannot see why. I just have "the usual suspects" but definitely no *BIG* files there. Already checked some potential folders but just a few MB there.

Also sorted the folder list by file size and checked the "big" ones ... nothing really BIG to see.

PS: I'm talking about files "on my iPad" here. And I'm still on iOS 12.4.1