I have a VXA-2 drive as well, Depends on what tape I use x10 40/80 or x23 80/160 as to how much I get stored on the tape.  I just always assumed the tape drive told SME how big of a tape was installed in it.  Hmm, now maybe not.  For my X23 tapes, I think I will test density 0x93 SDLT160 compressed. Right now my tape drive density is set to 0x81 DLT 15GB compressed.   Not sure what set it originally to that.