Definite effect it has upon uniformity, better growth, better management. if the day old chicks are nor well graded on the basis of weight, size, health then the flock will suffer till the end of its life.
small birds are not comfortable with large one and try hide under feeders, drinkers, corners of the shed, cant easily eat feed, and drink water. even remain small till end of life or die due scaring of feed , water, pecking by large ones. so they also harbour germs line e.coli due to poor health status.