I bought a bunch of chickies from the store and had two of the same breed, Blue Andalusians, one of them was growing really quick and the other was thin and tiny. She's light, but now she's a hen and she's still a bit smaller then the other Andalusian. But she lays eggs, and seems alright.
This happens sometimes. Don't worry.
I have a peahen that layed eggs and suddenly had peacock feathers after she molted. She's still laying eggs and is perfectly healthy.