My friend's kiddo (referred to above...) is truly just average in build. She's not husky and not skinny; she's just "normal kid average." Her shoulder width also seems average to me.
DN, on the other hand is very thin. She is 7.5 and is
teeny in build (though she
is tall!). She is an athlete (runner, basketball player, and gymnast), and has very noticeable muscles. I swear the bit of "weight" on her is all muscle because she looks like she weighs about 35 pounds, but is actually closer to 45-50. When I say she's thin, I mean it.
She's
teeny in frame (so narrow shoulders, too).
I think every kid is so different that it is hard to predict what will work best for any one kid. If one kid is 95% for height, and 50% for weight, but has a small frame, that kid will fit seats very differently than another kid who is 75% for both height and weight, but has a more average frame.