Hi Peter,
I seem to have located my issue, tell me if the same goes for you.
It seems to be that QuickTime Player X has the problem, not the exported videos, because when I play the videos in QuickTime 7 it is flawless, and again when I burn the exported files to DVD – plays no problem!
Please try playing your exported videos in QuickTime 7 if you haven’t done so already and let me know if you’re still finding the scaling problem.
Cheers
Tom