Please put subjects on your questions so we can follow the discussion.

OK,
Native storage versus compressed numbers are a marketing game.  The Magstar
K-Tape is a 40GB native (120GB compressed), yet I see data stored as low as
75GB and as high as 240GB.  It just depends on how compressable the data is.
If the data is already compressed, the size when it is stored to tape can
actually increase.  DLT is a much poorer compression performer than Magstar
so getting these numbers does not surprise me.  Are you using client
compression?

There is a another possibility.  You may have compression turned off, though
I doubt it.