The theory of short guys and muscle mass??
Well, look at Kai, he is 5'7 or so and outmasses Phil, yet Phil is 5'9 is already maxing out his physique as it seems.
Nasser is 5'11 and IMO he looks much better before adding all that mass to his frame, he was looking like shit in his later years. Flex Wheeler is the same way. So the arguement of height in some ways can be throwned out of the equation. Bone structure/joints have more to do with it.
The only guys that come to my mind right now that can add mass and still look good whether height is concerned is Chris Cormier, Dillet, Freeman, Ronnie of course, etc..