I wokled with some of these folks right after high school. Worked a couple years at an institution. they are usually shorter but a lot stockier than normal. Plus they do put weight on easily. that isn’t so much a problem till later years though as they grow more sedentary.
Yes, I know that in adulthood it can be difficult to keep people with Down Syndrome from growing obese. Little Trig doesn’t look fat at all. He just looks solid like my gorgeous daughter.