The IBM drives have a hardware compression and feeding them compressed data might well have an adverse effect.
I wonder why you use DAT72 tapes, because the 7214 comes with one of two possible drives: one for DAT160 (80GB) or DAT320 (160GB) tapes or an Ultrium drive for LTO4 tapes (800GB).
The drive you use (obviously the former one) is downward-compatible with DAT72 (and DDS4, for that matter, according to
IBMs product description), but you could simply switch to another medium and use its real capacity.
Btw.: a simple search for "IBM 7214" in Google revealed the mentioned link as first hit and the information about the drive options are near the top of the linked page. It is not forbidden to use Google prior to opening a thread here.
I hope this helps.
bakunin