I would say Rory looking strong is due to him training *smarter not harder* with some of the best in the business at cultivating wrestling strength.
GSP and Rory manhandle ppl and look really strong because they have the best training methods for developing that type of strength. Anyone could develop the strength GSP and Rory have if they only knew how.
Developing that kind of strength isn't as easy as doing squats or running a lot. You're using different muscle groups. If a person took the time to study human anatomy and recognized which muscle groups were most important in successfully executing a takedown. Then, experimented to figure out how to best target and develop those muscle groups to their full potential -- they could probably enjoy a big advantage over others who aren't willing to go to those lengths.
I think at some point GSP and his trainers may well have done exactly that. That's the reason why GSP's legs are so massive in comparison to everyone else's and its also the reason why GSP and Rory have such dominant wrestling and top game.