Exactly Sly, there are different types of muscle and fitness. Having a six-pack and being all toned isn't necessarily the best way to be as fit as possible as a sportsman. It may look nice on a beach trying to woo the ladies, but that doesn't mean it's the best for trying to play 50 games of top class football a year.
Unsurprising really that at the start of his career where he would wouldn't have been under a strict fitness program that he had a "looks" body, and that with each season at top club his figure has changed.