All the chicks in this picture are 7 weeks old and were obtained as day-old chicks via mail order. They are Delawares (white) and Buff Orpingtons (tan). The little Delaware chick has essentially not grown in size and she is still the same size she was as a week-old chick, although she does have wing feathers and some tail feathers. She still has down on her head and most of her underside. She appears to be healthy and eats/drinks normally. She seems to get around OK, but she is less than a quarter the size of the other chicks at this point. From what I can tell, the other chicks do not seem to pick on her.
I've never seen anything like this and was wondering what thoughts others may have on this. At this point, I can't really see her having a sudden growth burst, so I am not quite sure what to do with her.