It could just be that Dorn worked out a lot in the TNG run. But his build in Heart of Glory relative to Birthright Pt. II is noticeable.
I agree, though I think there is a considerable amount of padding on Dorn in Birthright in order to make him look more muscular. I seem to remember reading somewhere that Dorn was almost always padded, which is why (even with all the sex Worf gets to have on DS9) we never see what any part of him looks like under that uniform.