xor: The go to last cell thing is useful, but I already tried deleting everything outsize the active area before I posted this thread. It didn't do anything - whatever the default rows that Excel replaces deleted rows with are, they have the same problem.

p45cal: That was what I assumed at first, but I'm seeing basically the same file sizes with everything unhidden. I cannot replicate your result.

FYI, this is happening in Office 2010, v14.0.7192.5000 (32-bit). Perhaps that's changing things somehow?