I've been digging deeper on this issue because it's affecting my app, and the problem is clearly and issue calculating the size on iCloud. The information that I'm adding now it's already on the FB9770168, but I'm posting it anyway here so non-Apple employees can see the findings.
Size reported on my app's settings by calling allThumbnailsSize() shown above.
Size reported for iCloud Storage
Size reported by iPhone Storage (from the 216.6 MB, you'd have to subtract ~100MB from another database that I have on the app.)
iCloud size reported for the same app on my Mac:
As you can see, even if the 216.6 MB were true (which they are not, explained above), the iCloud size it totally wrong.
I even went a step further and decided to perform a test to see how much data was being downloaded from iCloud. What I did is delete the app from my device, reset the network statistics, let it download all the information from iCloud, and check the statistics again.
As shown on the images, if the iCloud size was correct, the download of all my data should have consumed ~757.1 MB. What it actually consumed is just ~78 MB, which is basically the size of the images reported in my app's settings + metadata of the object (small strings + bools)