Check which device files in /dev/rmt/ are all pointing to your DLT drive. Usually you have a device file with and without compression I/O. Also check the settings on the drive itself. You should always be able to override compression settings on the drive itself.
Check your tape system documentation.
The "compression" they refer to is
device level compression and even though
they may say 40/80GB, you can typically
expect an average of a 1 to 1.5 compression
ratio (60GB) after all is said and done.
There are many binary files which will not
compress well at all (in some cases they
actually increase in size). You can (as previously
suggested) use software compression such as
"compress" or "gzip" to be able to get more
data on the tape but if you do so, the
device level compression (if used) may end up
increasing the size of the data stored to tape.