Symantec Backup Exec 11d compression

Unfortunately, the data types you described are already compressed.
Your hardware and software cannot compress it any further. In fact, compressing compressed files actually increases the file sizes. Don't worry, Backup Exec knows better than to compress compressed files so you can leave your settings in place.

The compressed capacity of a tape drive is a marketing ploy by the tape backup industry to make them more appealing. Yes, if your backup consisted entirely of uncompressed files you might get close to 75GB but never 80GB. You can only count on getting the uncompressed capacity of a tape.

Three options:
Obviously, a higher capacity tape drive will resolve this problem.
You could also swtich to using two tapes but you would have to load the second tape when you arrive for work in the morning and your backup would have to finish during the business day.
Finally, if you have some data that does not change frequently you could seperate it from your active data. Backup the active data on your regular backup. Now you can backup the "archived" data manually with a separate backup job. Once backed up you only have to back it up again when new data is added to the archive.

I hope this helps.