This happened to me - an image I had placed was too wide so it forced the overall size of the email to a wider size. Try changing the size of your images to a smaller proportion to see if that's the culprit. Good luck!
I've been poking around for a few days now and I can't find out what IS the normal width of the templates. Is it 750? I want to make my images look as good as possible and not make the editor scale them at all. Thanks.