Updating old Blender-files (<2.5) to 2.7 -> size issue

Hello,

I’m currently maintaining my old blend-files (2.49) a little bit, and as soon as I save them, some of them gain a plus of up to 33% in size, but why?
Do the newer Blender versions put some hidden extras into them the users don’t see, or how can this be explained?

Greetings