Chicks grow faster in the winter. However, these are not winter chicks. These are fall chicks. Fall chicks are often, though not always, smaller than their winter/spring counterparts. There are various reasons for this. I fall hatched for years and my were not smaller than their winter/spring counterparts.
However, with that said, many people complain about fall hatched chicks being both hard to raise (this I agree with) and being smaller.
Typically, I find in my area of the deep South that the summer raised chicks are both the most difficult to raise and are the smallest of birds; which is why I stop hatching by April. They are just too difficult to rear after that.