Actually no, Karl, they are not necessarily bodybuilding. They are often used for purposes other than bodybuilding. If you lift heavier and heavier weights, you may be powerlifting, you may be olympic weightlifting, or you may be cross-training to become more effective in a particular sport. Bodybuilding is not necessarily your goal. You might be trying to gain strength and muscle to compete more effectively in a higher weightclass---as Evander was---because he was trying to go from Cruiserweight to Heavyweight where the big bucks and big matchups were to be made. Bodybuilders attempt to add muscle, reduce fat, and gain definition for the purpose of developing a more muscular physique...whether it be recreationally or for bodybuilding competition. For bodybuilders, the physique is the most important goal. For others, the physique is secondary. Evander's motivation was to move up from cruiser and become the heavyweight champion of the world, and once he achieved that he wanted to remain the champ. Later, his goal was to regain the championship. He wanted to be bigger, more powerful, and stronger. He was a small heavyweight, and he was at a disadvantage in comparison to the naturally bigger men. He didn't give a shit about striated glutes or a Christmas tree lower back---he wanted to take on the big boys. Period.
Top end Sprinters generally have about 90%+ of the muscle mass as drug free bodybuilders do for their frame size (genuine stat, not pulling that out my ass). Shot putters actually tend to have MORE muscle mass than drug free bodybuilders for their frame size (the extra fat allows that). Both train with weights and gain muscle mass as a consequence and prerequisite of getting WAY stronger and more explosive. But THAT's their goal, neither is bodybuilding. They'll follow a very different procedure in terms of exercise choice, rep range, frequency, volume, periodization etc. It's different. Unfortunately boxing is still in the dark ages in these things and Klits & Haye aside, most probably do bodybuild. Holyfield did from everything I've seen of his weight training programs.
Klitschkos have been reported as gaining weight in training. Just word of mouth from James Lampley, nothing official.