LT0-4 Tape Capacity Problem

We are running Backup Exec 12.5 and have recently moved from LTO-3 to LTO-4.  All works fine except for the tape capacity. All the data is precompressed so we do not expect more than 800GB but tapes are being ejected at between 600GB and 650GB during backup as full! For example, with new HP media, looking at the statistics, the following is reported:

Used capacity: 780GB

Bytes written: 584GB

Bytes read: 192KB

Mounts: 4

Seeks: 15

Errors: All  0

Ideas?

Thanks

Michael