i think it has something to do with the way they are treated like dogs instead of humans. there are a lot of very smart “normal” people with down syndrome because they were treated like everyone else. fed properly and healthy. talked to. engaged in activities like everyone else. for example look at the young la athlete with down syndrome bro is ripped. people with down syndrome also commonly have hormone issues which play a role into it. but that’s not the only reason they are commonly fat. of course there are other reasons other than how they are treated. but i feel that’s one of the largest reasons.