On my Mac I have 12GB in all Devonthink files. But on my Iphone and iPad I am shown 64GB, which makes all the memory full. I also only noticed it because the device memory is full.
Where is the difference coming from?
- Do you have DTTG2 still installed, or did you only recently remove DTTG2?
- Has the sum total of your databases been larger than 12 GB in the past (e.g. have you recently removed large files from your databases)?
- Are you using the same databases on all devices (so do you have any additional databases on the iOS devices which aren’t on the Mac [I know, pretty silly question, but might as well ask…])
- Stabbing in the dark here: do you have “backup data to iCloud” turned on in the preferences?
- Where are you reading the 12 GB and 64 GB values?
What kind of sync are you using?
- Hast du DTTG2 noch installiert oder hast du DTTG2 erst vor kurzem entfernt?
No
- War die Summe Ihrer Datenbanken in der Vergangenheit größer als 12 GB (zB haben Sie in letzter Zeit große Dateien aus Ihren Datenbanken entfernt)?
No
- Verwenden Sie die gleichen Datenbanken auf allen Geräten (haben Sie also zusätzliche Datenbanken auf den iOS-Geräten, die nicht auf dem Mac sind [ich weiß, ziemlich dumme Frage, aber könnte genauso gut fragen …])
No
- Stechen Sie hier im Dunkeln: Haben Sie in den Einstellungen “Daten in iCloud sichern” aktiviert?
Yes
- Wo liest du die 12-GB- und 64-GB-Werte?
They are shown in the system. Where else 
icloudKit
They are shown in the system.
Screen captures could be helpful.
Just for completeness: what is the reported size of the app and documents & data when you tap on DEVONthink in the screen which you captured in iOS?
Are you indexing folders? The indexed data won’t count towards the size of your databases on the Mac but will be transferred to DTTG IIRC (because DTTG doesn’t/can’t directly support indexing); that could explain a significant difference in size.
A screenshot of File > Database Properties… would be helpful, especially if your database contains indexed items.