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late fall (oct-erly Nov) hatched bantams tend to be smaller, they also tend to have better wing length, and tighter feather due to cool moist environment. they will lay a slightly smaller egg. Additionally only set the smaller eggs, try to cut to smaller birds by weight use an electronic scale and weigh them use only smallest birds, get fall hatches for yourself and test mate prior to pairing for fall matings. There are a few other tricks that work all will take a quite a few generations to set size.
The size of the egg does not always effect the size of the full grown chickens, just the chick size. A tiny chick can grow just as large as a larger chick can.
The best way to get consistently smaller chickens is to breed the smaller breeding stock to each other.
The winter hatched hens can produce offspring that grows large, but selecting the smallest of the babies that hatch each generation will increase your chances of consistently smaller chickens.