Yates only had been training for 6 years when he won his first O, so he didn't really have muscle maturity. When I think of muscle maturity, I think of Lee Priest's shoulders, Jay Cutler's Legs and Ronnie Coleman's traps. They have that really solid outline with excellent muscle separation, but the actual muscle fibers are no more or less grainy than the rest of their bodies.
Just my understanding of muscle maturity. As I get older, it is muscle separation that becomes easier to achieve, there is no difference in the quality of the muscle and size becomes harder to gain and maintain.