In today’s online chat with Asafa Powell, he stated that he has lost 4kg of body weight since last season. The appearance of increased musculature may thus simply be a result of improved leanness.
BTW: His first 9.72 was in Lausanne, not Rieti: http://www.youtube.com/watch?v=B2QENrUORZg&feature=related. I love the camera angle at 4:39. It gives you an idea as to how fast 9.72 really is.