iCloud sync eating up drive space

I’m using Devonthink Pro Office on a Mac Mini and Macbook Air, and I’m trying to get them to sync through iCloud. There don’t seem to be problems with the sync itself, but most of my Macbook Air’s SSD is now taken up by Devonthink database files. I have the databases in a folder in my user directory (about 70 GBs across 8 databases) but there is also a 60 GB folder for Devonthink in the Mobile Documents folder. (Can’t see this through Finder, but Daisydisk shows it.)

It looks like I’m effectively having to store two copies of every database I sync. Is this the way it’s supposed to work? It seems pretty unsustainable.

It’s not storing two copies of the database, as sync doesn’t merely copy databases.
You can move your databases to an external portable drive to reclaim space.
Also, the sync data should be reclaimed after iCloud has done its own syncing.

You could try to thoroughly verify the sync store (see contextual menu in Preferences > Sync), this forces iCloud to free as much disk space as possible.