Just thinking out loud (not prototyped/tested) - is it possible that both servers have different hard drive partition configuration when they were formatted as part of setting up of the servers (sector sizes, etc)?

I'm guessing the when restoring from a compressed backup, a different drive configuration has some role to play in the final size.

By the way - closing one of my action items from a previous comment - I was able to confirm that the compatibility level has nothing to do with this issue.