I have noticed this on my birds, I have a 12 oz. hen, at 2 years old, she is still slowly maturing. even since last year when I got her she has gotten more upright, has a larger chest, and is appearing to have a shorter back!
I have some pics of chicks that are from @shahtir101
This is from Rocco, I can't remember if she posted a pic of him.
This is a very blurry picture of another rocco baby
This is another rocco baby, Poses better than this but she was sleepy
This is a truffles baby, I think she is the...