I wonder if DJ181 and Tdongz can explain to us why Jay Cutler's quads are so much larger and more developed then this guys. It's all about progressive resistance and the more weight you can move the bigger your muscles.
Max squat is about the midsection. Look at his waist, it's big and strong. That is what is allowing all the magic to happen.

When we go down in weight, lets say 70 % of the max, then the legs become more important. Not to mention the differences in drug use and personal differences, person who squats 800 pounds might not be the biggest guy around but he is bigger than when squatted 400.