The database only stores the last 20 models, but that itself is minimal data. Each backup will have its own 20 models though, but again this is tiny and you can delete the older backup archives, it is the Data Folder that holds the large quantity of data and this is all Filesets, from when the backup first ever ran, until the latests backup.
Probably the easiest thing to do is:
Copy the current backup folder to an external storage (wise to do this anyway)
Remove it from the server
Purge older Filesets no longer used, using FileWave Central
Run a fresh backup
At some point in the future, when happy, remove the externally stored copy (consider copying the latest backup instead)
Identifying Filesets by size can be done with a query. This query has:
'All Devices' > 'Device Name'
'Fileset' > 'Name'
'Fileset' > 'Install Size'
'Fileset' > 'Fileset ID'
'Main Component' > 'Fileset'
Then order by Install Size.
If you add the following Criteria, I think that should list Filesets not used:
Worth doubling checking before deletion. If you highlight a Fileset and choose Report, that should show all current status reports for that Fileset, which will be blank if none.