Database backups performed with the Backup Exec Agent for Microsoft SQL are observedly less than the combined total of the .mdf, .ldf. and .ndf files.
It is normal and expected for database backups performed with Backup Exec through the Agent for Microsoft SQL to be significantly smaller in size than the combined total of the sizes of the respective SQL database files.
To confirm that the data being backed up with Backup Exec is the correct amount, perform a dump of one or more databases with SQL, and then compare the size of the data dump with the size of the Backup Exec backup. They should be approximately the same size.
An alternative way to confirm that the data is being protected correctly is to simply perform a restore of the database(s) or SQL server(s) in question. Restores can be redirected to another system so as not to disturb a production environment.
Note: Symantec recommends that test restores are periodically performed, just to familiarize yourself with recovery procedures and iron out any issues that may arise during the process.