All our kids were scrawny. When small they were all in the 3rd percentile for their age. Our pediatrician just got used to it, but every once in a while he changed nurses and we had to teach the new one that it was okay, and that our kids were healthy.
David was one of the scrawniest. One summer day, he must have been 5-ish, he came in after playing outside all day. Mildred looked down at his legs, which were pretty much covered with bruises (as little boys can do), and said "David, look at those legs".
He looked down and said "Yeah, they're fat, aren't they" :)