Hi! Yep, unfortunately the way that data is stored in iCloud’s CloudKit service is very inefficient in its use of space. The backups do indeed get that large.
Which is one of the reasons why I’m working on a replacement backup system - still iCloud based, but using compressed JSON files on iCloud Drive, rather than iCloud’s CloudKit database service.
The new system allows me to ensure that the backups stay a sensible size. And has the added benefit of the backups also being inspectable via iCloud Drive.
I’m hoping to have the new backup system ready within a few weeks. Unfortunately it’s meant that I’ve had to delay work on Arc Mini and Arc v4, which sucks. But the current backup system just hasn’t scaled well to the sizes of our databases, so it’s time for it to be replaced.