Here's my guess
the big chick stole all the growing genes from your little chick. Your only normal chick is the one in the middle. The other two are mutants
JUST KIDDING!!
After seeing the size they were when you first got them it makes a little more sense, maybe. That big guy is older, at least a week or two than the other ones and the middle one could be a week older than the small one. Which if you want to get 'technical' about it they were all born within a week of each other, um kind of. I can see that the little one does have feather growth, and it looks like it is catching up to the middle chick, just not as tall. That happens, even if they are not banty chickens. Variation in size happens with all of us.

JUST KIDDING!!
After seeing the size they were when you first got them it makes a little more sense, maybe. That big guy is older, at least a week or two than the other ones and the middle one could be a week older than the small one. Which if you want to get 'technical' about it they were all born within a week of each other, um kind of. I can see that the little one does have feather growth, and it looks like it is catching up to the middle chick, just not as tall. That happens, even if they are not banty chickens. Variation in size happens with all of us.
