Anybody think he ever did a leg extension in his life? All squats and probably heavy leg presses, using good form, with progressive overload to the point of using very high weight. The body must adapt.
(not to mention this guy probably doesn't eat the calories a bodybuilder does in a bulking phase, and he does do a large amount of high intensity cardio as well.)
These guys are all sprinters of sorts... and generally like 100 Meter runners they are supposed be to be built for short distances with explosive power and explosive powe does requires extra bit of muscles!
Just like 100 meter runers will have very muscular built compared to long distance runners these guys have muscular built compared to say for instance Lance Armstrong and others doing Tour de France!