Maybe it's just a smaller birds, maybe it's a bantam. I have mixed flock and with my smallest bird, I can fit her whole body in one of my hands, she's still a productive layer, her eggs are just really small, and she is tiny.
Are they all the same age? Are the others combs and waddles bigger, I'm still not quite sure what you mean by "fully grown" the pullet pictured would only be 4-5 months correct?