MPEG FULL SCREEN ISSUES

[mbaruffa]

Why do some of my videos display correctly at full screen and others 'stretch' horizontally?

They are all Mpeg-1 0% comporession
varied frame rates... 30 or 29.97... with a try at 25 fps...
data rate at 2048k,1123k,2250k...
All stereo 44.1 audio either 128 or 224 bit rate

The software I use to generate the videos is Ulead's MediaStudio Pro...

Thanks!

Mike