It's ALL muscle, ladies.
[ANOTHER CONTRIBUTER] To expand just a bit on this....While appearing as if he's just an overweight slob, the truth is that he's been measured by doctors and they found that it is almost entirely muscle. This isn't as far fetched as it might seem...go watch some videos of Professional Weightlifters...to see them on the street you might mistake many of them as 'fat'. People who build up muscle primarily for strength do not look the same as someone who builds muscle for show...this is why Arnol Swartzeneggar in his prime, might have looked better than a professional weight lifter but would never be able to match their strength. It's all how you excercise....Certain weight training builds definition, (this is what most body builders practice) while other excercises build strength alone (this is what professional Weight Lifters....or Professional Sumo Wrestlers build). Now Kingpin's abilities are a bit different than what one would expect in real life, because in addition to having peak human strength he is also quite fast and flexible...traits not normally associated with someone who build's enormous muscle mass for strength alone. Still though he lacks any exceptional agility...which is what characters such as Daredevil take advantage of when fighting him....even then they often find him more than a match, however.