Originally Posted by tennis_balla
Ash posted this a while ago, and I think it demonstrates perfectly the answer the OP is looking for.
based on the picture, simply observe that each player is taking the racquet back at different points.
Rafa is taking it back quite noticeably less than Sharapova. This means he can use more time to calculate the shot etc, and then use his sheer muscle to bring the racquet around.
Sharapova needs to bring the racquet back slightly farther than Rafa because she simply does not have the muscle mass that Rafa possesses.