In SQL Server, the transaction logs DO NOT truncate unless they have been 1) backed up or 2) a TRUNCATE command has been given.
A log backup is different from FULL or INCREMENTAL.
Also, check to see what the setting is on the SQL Server itself. Is it set for autoshrink? (right click on the database, select properties, go to Options, make sure Auto Shrink is checked.
Backup Exec is probably sending the TRUNCATE command, but if the database isn't set for autoshrink, the space will not be reclaimed.
-SQLBill