All right, so… When you right click on the exported file (from your desktop) and click properties > summary, what do you get ?
Are you sure that it does fill your preview window in Premiere? Try zooming out to say 25% (I’ve seen it happen before 😉
Can you try exporting a few minutes to uncompressed and see if the same thing happens in your media player ?
You can try importing it in a new project, but I can’t imagine that it would solve the issue.
BTW, does the image just appear centered with a black border ?
Vince