Hi,
I had this exact problem myself and it took me a very long time to figure out. If you are using FCP which i presume you are the problem is with the default setting in your options after you go into export-using quicktime conversion – options – size. The problem is that for some reason FCP sets this to a 4:3 as default. I figured out through a friend that if you go into dimentions here at the bottom of the list is custom. click on this and change the dimentions to 704×400. DO NOT tick the preserve aspect ratio or the deinterlace source video.
This should work for you, to be honest im not a computer whiz so i dont know why, but i hope it helps.