Strange backup set

[Ivakin Dmitry]

I have some problem with backup set. I use DLT tape 40/80GB. For example, I check the Windows folder, the size about 47GB. I'm sure (I checked it before) I can upload to tape about 54GB.
But..when I start backup, everytime the ntbackup told me, that not enough space on the tape. I checked backup size during the ntbackup running, it's 58GB!!
How it can be? May be somebody already meet this problem before? I can sure that no compression on hard drives, no other functions. I used hardware compression on DLT.

Please help,

Dmitry.