I have had a similair experience before. I had a little chick that I hatched, and I named him Tablespoon. He would always be the first one to the food, and always chirping loudly unless he was being cuddled
. He feathered out a little bit on his wings, and then did not grow any more, did not get any more feathers, while his other siblings got bigger and bigger every day. When I needed to put the chicks outside because they were outgrowing their small inside rabbit cage, but i kept her inside because i was afraid that she would be cold or get smushed. I kept her inside with a sibling so he didn't get lonley, his roomate was a barred rock, and very gentle with him. She would wiggle himself between the wing and body of the other chick, he loved to feel cuddled. Sadly, while outside (It was a very very warm day) with his larger siblings, playing, he was smushed by the larger chicks under the shade of a box and died

Another little problem besides Tablespoon's small size, was that he got food pieces stuck in his nose, how he did that, I don't know, i was just worried because he breathed through his mouth, not his nose because it was plugged, and i was afraid he would get an infection or something.