This solution applies to Barracuda Backup Service.
When you perform a full backup of your SQL instance, the Barracuda Backup Service will send a command to SQL to let it know to truncate the logs. SQL will then write all zeros to the existing log (it will not change the size of the log). When new data comes in it will then overwrite the zeros with the new data.
It is possible, after a long time, the SQL log file could grow to unwieldy size. If this should happen, please refer to this Microsoft article on how to manually shrink the log file(s):
Link to this page: