bantam sized barred rocks?

[spish]

i have 4 barred rocks that i hatched out last year, and cant quite believe the size difference between them. all 4 are pullets, all 4 are now nearly 8 months old. 2 are your average size pullets....but 2 are tiny!!! like bantam size tiny, and they even lay tiny little eggs! they came from the same parents but the size difference is amazing (shall get some photos up shortly when i get outside)

what could be the cause??

 

[Chickenrandomness]

must have been a genetic thing, could have been some bantam genes floating around in the gene pool