Nonstop backup file size too small?

I've been experimenting with nonstop backups with ATI2012 in Partition (or is it Disk) mode. i.e. it is backing up the entire drive not individual files.

What's worrying me is the size of the resulting backup file(s).

The size of a .tib for a normal image on this disk at the moment is 27Gb.

But the sum of the files in the hidden "Time Explorer Storage" file used for non-stop backup is much less than this, at 20Gb.

This was measured at the point after the nonstop backup had first been switched on, and a few hours later.

Is there an additional hidden folder somewhere that I'm missing, or is this cause for concern?

Other than doing a full image, then restoring from the non-stop as a test to see if everything restored correctly, I'm not sure how to check all is well. And I really don't want to do a test restore if I don't have to.

Any ideas?