Backup issue

Hi,
I have a Compaq Proliant server with NT 4 runnings. HDD size is about 20 GB. Partitioned into 2 drives of 16Gb (D) and 4 GB (C).
I have the OS in C and Data in D drive.
I have a DDS3 tape drive to a backup data.
I have about 15.5 GB of Data now.
When I try to use DDS 3 12/24 data tape for backup, I am able to backup only about 13GB of data and then the system prompts for a new tape.
Why is this so?
HOw do I make use of the “Compression” fearture to save the data?
Any help is most appreciated.
TIA.