Hello Spansos,
Easy answer, Ignore the capacity. Its not a foolproof number that you get.
Some tape drives can accurately report capacities.
For the rest that can't, BE uses educated guess's from when you did the last backup. So if you last backup to that tape only put 16 gigs on it, BE will report that as the size.
Try running your backup and verify this is the case for you.
Hope this helps
---cheers
shirmal