Run that backup to disk first, and then duplicate it to tape.
If you want to find out which folders are big, you can get an application on the net that checks for folder sizes. Not too sure of the name/s, but if you do a search on Google I am sure you'll come across one.
You can also look at excluding files like *.mp3, *.wav, *.avi etc. That adds to space on the tape and also time taken to backup the files.
Laters!