As far as powerlifters are concerned, it's far less of a factor of how much muscle their bodies carry. but how balanced their training is. The goal in powerlifting is to get the highest total on the three competitive lifts: squat, deadlift, and bench press. A more intellegent approach is to achieve this goal is to become as strong as possible overall and near competition time, do some specialty work so that you peak for these particular lifts. This requires keeping balanced work along the natural strength curves of the body, keeping in mind not only the agonist/ antagonist muscles, but also the general proportions of the body as well.
Unfortunately, far too many powerlifters will not take this balanced approach and will only train for the three lifts and do these and certain support movements. They will rarely, if ever, put in any real work on any muscle groups that they don't feel are key to their lifts. Nor do some of these guys ever do heavy high rep work. Doing high rep (20 reps +) work to failure with relatively heavy weight develops both cardiovascular density and support within the muscle and far greater muscular endurance (this is why, while I can max out on the squat at over 800 lbs, will far more often rep out at 500 for 20). This not only helps add by giving support to their single rep max, but also greatly enhances their athletic ability.
Worst yet, many guys who call themselves powerlifters will often specialize in one lift and do not keep a balance between the three lifts. This most often happens with the bench press, but can occur with any of the lifts. There are many guys out there who can actually bench more than they can deadlift of even squat. This is way off the natural strength curve for the muscles. IMO, if you are someone who just enters bench pressing contests, you may be a strength athlete, but in reality you are not really a powerlifter.
For the reasons listed above, I actually consider "old school"-style, hardcore bodybuilding superior to powerlifting as a training method. In this more basic version of bodybuilding, the goal is to fully develop every major muscle group along the natural strength curves in proper proportion. Also, you develop every aspect of muscle (strength and endurance) to it's highest potential. This type of BB training also requires a significant amount of work on cardio and flexibility as part of the training. Like the powerlifters, too many bodybuilders overly specialize and become "pumpers" who work too much on isolation exercises with too much emphasis on lighter weights and high reps, though.
Before the drug explosion of the early '90's (prior to this, there was a drug problem in powerlifting and BB, but it was comparable to that in many other sports. It was just more obvious than football or baseball, because these guys looked like they "had to be on something". After the early '90's, the drug use became insane among many of the top competitiors and the wannabes), it was very common for bodybuilders and powerlifters to cross compete in each sport and it was the norm for almost everybody to at least cross train. Some guys' genetics would be too far from what is neccesary for success in the other sport, so they might not bother getting into competition (too small of a hip structure and waist hurts you in the squat,but too wide hurts your taper in BB), but almost all the high level guys cross trained in the other sport in the off season.
As far as your brother is concerned, it's probably far less his actual weight than it is his training methods. Unless he is doing an extremely high level of drugs (steroids, growth hormone, ect.), his body should not create more muscle than his frame can handle. (If he is doing that, obviously these are the least of his problems.) The wear and tear on the body from improper training methods and overtraining certain muscles and leaving others undertrained will definitely take it's toll. Powerlifting in general will give your body a real pounding over the years far more than "just carrying all that weight". Especially if that weight is lean muscle and not fat, it will carry itself.