It's not just you, and I think the difference you see in that regard are true to the movie designs. I believe there was a deliberate attempt with thee MkIII design to make him look "heroic" with real broad shoulders tapered down to his waist but they moved away from these exaggerated proportions for the Iron Man 2 armors which I think works better. But I still love the MKIII because it was the first red and gold armor in the movies and reminds me of the classic comic version. With that said, I also can't wait for the MkVI, it looks absolutely perfect.
Mine will be here Friday!