When you perform a full backup on a database, it also backs up the transaction log. Your log file size has grown to 20 Gb, becasue you have not shrinked it:
Netbackup will not shrink the transaction log after taking a backup with truncate option. It will only logically truncate the log file . You need to run DBCC SHRINKDATABASE or DBCC SHRINKFILE command in Query Analyzer (SQL Server) to reduce the PHYSICAL size of the log file.
Here is a MSFT article explaining the process:
Relevant snippet from the MSFT Article 873235: http://support.microsoft.com/kb/873235/
Shrink the transaction log file
The backup operation or the Truncate method does not reduce the log file size. To reduce the size of the transaction log file, you must shrink the transaction log file. To shrink a transaction log file to the requested size and to remove the unused pages, you must use the DBCC SHRINKFILE operation. The DBCC SHRINKFILE Transact-SQL statement can only shrink the inactive part inside the log file.
You need to run DBCC SHRINKDATABASE or DBCC SHRINKFILE command in Query Analyzer (SQL Server) to reduce the PHYSICAL size of the log file.
Netbackup backs up and truncates ONLY the logical part of the transaction log file, but the Actual shrinking part has to be done on the database itself.