I would have to guess they are referring to being mature enough for show season. When you hatch a bird does not change its genetics. I hatched my orps over all seasons over the years and have never seen a difference in size when they are mature. Of course I don't consider a bird fully mature until around 18 months. By that time, regardless of when you hatched them, they should fill out the same as any others, as long as your breeding is consistent and results typically are as well. Pullet eggs are the only times I have seen birds start out small, but again by about 18 months they are normal size, because it is driven by genes not the size of the egg or time of year you hatch.