It’s certainly the top suspect, but compact depends on many things including your specific data behavior. Because the default 25% wasted space threshold isn’t hit all the time, you may also see usage fluctuate.
The COMPACT command and no-auto-compact show options for Compacting files at the backend whose goal is to reduce space usage at the expense of downloads and uploads, thus there can be cost tradeoffs. Backblaze B2, I believe, charges for storage and downloads but not for uploads. Other services may vary.
Does duplicati download any data?
BackBlaze is reporting large daily downloads when using Duplicati
A log-file, for example at Information level or better, gives reports on why Compact does or doesn’t run.
Viewing the log files of a backup job will do if you prefer clicking. Look for the Compact section for data.
EDIT:
You have to go up from Information to Verbose level if you want all the numbers in the log file. Example:
2020-03-03 12:42:05 -05 - [Verbose-Duplicati.Library.Main.Database.LocalDeleteDatabase-FullyDeletableCount]: Found 0 fully deletable volume(s)
2020-03-03 12:42:05 -05 - [Verbose-Duplicati.Library.Main.Database.LocalDeleteDatabase-SmallVolumeCount]: Found 3 small volumes(s) with a total size of 17.44 MB
2020-03-03 12:42:05 -05 - [Verbose-Duplicati.Library.Main.Database.LocalDeleteDatabase-WastedSpaceVolumes]: Found 17 volume(s) with a total of 25.24% wasted space (581.57 MB of 2.25 GB)
2020-03-03 12:42:05 -05 - [Information-Duplicati.Library.Main.Database.LocalDeleteDatabase-CompactReason]: Compacting because there is 25.24% wasted space and the limit is 25%