Why is Inky so small?

[BarbG]

I have seven 4-week-old Black Australorp chicks hatched on the same day at the same hatchery that are all supposed to be female. All seven look absolutely identical with regards to feathering, color, shape of the tail and wings, feet, combs, etc. but one of the seven is half the size of the other six. "Inky" is as active and healthy as the other six, appears to eat and drink as much as the other six, and is not being picked on in any way, yet she is really small. Interesting enough, she is the only one who will fly up onto my hand - the others will run away. Any clues what might be going on with such a size difference?

 

[Tiramisu]

Chicks all mature at different rates

 

[Buff-Island-Australorp]

I have (5) BA's and one has MAJOR cross beak. She is much smaller than the others, and it's not so much her body that is smaller, but her legs are shorter. She actually waddles like a duck when she walks.
Although I am concerned for her, nature will take its course. Right now, she hangs with the best of them and has no problems.