Without knowing the specifics of the original file it's hard to comment but it sounds like it's anamorphic. In FCP and with the timeline active go to sequence settings and check the "Anaamorphic" tick box. Does it look OK now?
The original file is a 720 X 576 .mpg file. I'm using MPEG Streamclip to convert it to .mov (same 720 X 576). The result is a 720 X 576 .mov file where my original content is squished horizontally and there's a black bar on each side.