One major difference between chicks brooded by a momma and those under the heat lamp is that the chicks with momma usually spend more time during those first few weeks in the cold than do chicks raised under a lamp, which spend ALL of their time under heat. At least the chicks with momma come out and wander around a fair amount. Being exposed to the cold makes them feather in more quickly, which might mean their muscles have to develop more quickly. Just a thought.
Looking at them, the outdoor chicks appeared to have less feather development but maybe they get more down feathers.