SQL 2008 compressed backup waste 50% of tape size

I have tapes assigned for backup policies of SQL 2008 databases, the backed up databases are compressed with the compression option in SQL 2008 server.

the problem is that the tapes used for these SQL databases backup reaches full status when it's about 50% of the actual size of the tape.( Example : 1.6 TB tapes are full when it contains about 700 GB).

i don't understand what is the reason of this behavior, while this problem didn't appear when i was using databases backup from SQL 2000 server or normal Flat files backup. it takes the whole tape size (almost 1.6 TB).

So, Kindly i need your help with this issue that waste almost 50% of my tapes size