a small roo who's already got mature feather and learning how to crow

[CCCCCCCCHICKENS]

you got a point, but his sisters are as big as him, and he got one brother that's taller than him

Do they have the same parents?

 

[Epent12]

Do they have the same parents?

i dunno, my girls lay their eggs randomly

 

[CCCCCCCCHICKENS]

i dunno, my girls lay their eggs randomly

Well then we are back to apples and oranges. If they don't have the exact same mom and dad you can't expect them to all be the same size.