These are all factors that could affect the backup size.
1. The size of the database
a. The total number of managed systems and accounts.
b. Number of User IDs being managed
c. Log files
2. The amount of data stored on the appliance
a. Reports batch reports
b. Session Logs
c. Archive settings
If any of these changed it could change the backup size. The more information stored on the system the larger backups will get.
Reviewing the session logs listing tab shows Sessions Recordings that have been created but not yet Archived. Selecting the recording and clicking Archive Now will save the recording to the specified Archive Location.