They've proven that size of muscles does not slow down a person if they are the right kind of muscles. If you work your ass off you can still have alot of muscle mass and be in good shape. Jerry Rice was 6'2 200 pounds of muscle, and was in the shape to be competitive in marathons if he had wanted long distance running sport. The reason why so many guys are 270 in the nfl and fast is because they are naturally large now so they can be 270 and have 4-6% body fat which means high muscle to body weight ratio which helps maintain their speed.