Is the D2T a regular copy type backup, or is it a duplicate.
Only thing I can think of is If it is a duplicate, BackupExec will not attempt to compress it, assuming that if you wanted compression, you would have compressed the original B2D job.
This is only a guess, tho, so you'll need to wait for a response from a Symantec employee. (if they don't respond within a day or so, you could open a new thread pointing back to this one)