I help out with a High School mountain biking team; it’s a team #1 rode with last year. (He might be first special needs student to compete in a NICA high school race, though I wonder about the Utah league.) It’s a sweet deal — I do adult things, carry the first aid kit, and help with team communications; in turn I get to ride with a great group of adults and kids while I get quality coaching by listening and practicing with our riders. #1 helps too, I’ve made it part of his post-high school “transition” training.
One of the things I've heard from the coaches is that heavy kids tend to do well with mountain biking. The bike takes a lot of weight off knees and hips and, by necessity, these kids have powerful legs. Weight isn’t a big disadvantage on the downhills. Being heavy does make climbs harder, but that work boosts conditioning. Heavy kids who go to practice lose fat and gain muscle, so they improve faster than slimmer riders. That rapid improvement is a powerful reinforcer. If they persist they keep the powerful leg muscles and lose fat — and become quite competitive.
I see the logic. It would be interesting to get some data, but I couldn’t find any studies in the medical literature. It could be a fun research topic.
 He may be the only special needs student, so far, to compete in high school mountain bike races.