HP DAT72 Backup & Windows Backup Utility

dear all

recently we bought a HP DAT 72 Backup tape drive, in order to backup our 50GB data. so i used the windows 2003 server backup utility to backup our data into 72GB Tape. surprised the windows backup utility informed me that the data won t fit the tape, so may be we need more than one tape to backup our data. my boss asked me embarrassing question, how 50GB data will not fit 72GB tape? really i could not find answer.
anybody help to find answer. and how i can backup our data in one 72GB tape?