The sizes shown for individual databases, for DEVONthink To Go as a whole, and in iOS’ Settings can differ. Here’s how they relate to each other:

1. The Info popover for a database shows the sum of the sizes of all documents in the database, regardless if they are downloaded to the device or not.
2. The Help popover shows the total of the data store folder on the device, comprising of the downloaded document files, the metadata database, the full-text index database, previews, thumbnails, all the files needed by sync, and some temporary files.
3. The iOS Settings combine the storage size used for all files in the folder shared by the application and all its extensions (the ‘shared container’), the files used to provide documents to the Files app, and the Spotlight index.

If you have just moved from DEVONthink To Go version 2 to version 3, the storage size shown in the iOS Settings also add the size of the old data store left behind by version 2. It will be cleaned up four weeks after launching DEVONthink To Go 3 for the first time.

Note that iOS is using the APFS file system. Copies of files and folders (‘clones’) are counted with their size but don’t take up any space unless one of the copies is modified. File and folder sizes on APFS therefore don’t necessarily represent the real number of bytes used.

DEVONthink for Mac always uploads complete databases. You can exclude indexed items, though.

DEVONthink To Go always syncs the metadata for whole databases but you can select for the location, the database, or individual groups if you want to sync their content files too. If you sync only the metadata you can download the actual content later on demand.