I think the difference in size between hatchery stock and home hatched stock is mostly related to nutrition and living conditions of the hen. B/C, if I buy hatchery chicks from any hatchery, and raise those chicks, and set eggs from those birds the following spring, all things being same, and assuming that the hatchery has not had any degradation or improvement in their stock... If my chicks are bigger, then, nutrition and better living conditions have to be the reason. It would be highly unlikely for any study to be published, because it would not be in the best interests of the hatcheries to conduct such a study, and I doubt that any BYC hatcher is going to hatch X breed of chicks from hatchery A, and order the same breed from the hatchery in the same year.