Sounds like the tape is filling up.
What kind of drive do you have, what kind of media are you using, how much data has been writen to the tape before it ejects?
Remember that 2:1 compression is a marketing thing, not an engineering thing ( See http://seer.support.veritas.com/docs/199542.htm ). I usually figure about 1.3:1 as a Rule of Thumb, so a 20/40 system I would expect to get about 26-27 GB