You aren't "built" with fat. You are stuck with your skeleton, but you can be more/less muscular and have less fat with diet and exercise. Why do you think virtually everyone below heavyweight is so lean? Smaller guys just have no fat according to their body type? No, they follow strict diets. Cormier is still a little pudgy because he just now started cutting down to that weight and he never wanted to commit to doing so in the first place so he is doing the bare minimum to get down to weight. If he wants to improve he can still lean up some more and I wouldn't be too surprised if he does over the course of his career at LHW if it lasts very long.
The reason Cain is pudgy, again, is not down to his frame. If you wanted to say he was naturally that way, it would be down to metabolism, not frame, first of all. He is the way he is because of, again, diet and exercise, or more specifically, the lack of the former. There is an advantage to being able to eat all that you want and that is that he is able to train harder, longer, in many cases. His body is constantly in a "fed" state, meaning he is always meeting his caloric needs with his diet. If he were dieting to lean up, he would lose the fat but he would also have a bit more difficulty mustering energy because he would be relying on fat stores (thus losing body fat) to meet the caloric needs he wasn't meeting solely through his diet anymore. Once he got lean, he would have to balance his caloric intake with his energy expenditures perfectly and that can be difficult to pull off. As it stands, he enjoys the freedom to eat whatever he wants and have no worries about making weight. If he were bigger, like Brock or something, he'd have to be more concerned. But the fact remains that he could definitely change his body fat percentage if he dieted.
