I've read about one hen being much smaller after a couple weeks, but this is now going into the 18th week. I have a buff orpington hen that can walk underneath the other hens she hatched with without having to duck. She's not much larger than a 2-3 week old hen. She has flight feathers but still some down. In all respects, she appears healthy other her small size. Eats well, active, no issues other than size. Any thoughts?