Have been discussing this with friends recently - difference between "way-back-when" and today and today's chickens. Particularly their size being smaller now and not necessarily fulfilling their production potential.
No, I don't think that farmers separated their chickens. Poultry fanciers who showed birds did I am sure, but not regular people keeping chickens for food to live on.
But the environment is different now too. Back then, chickens were livestock and treated as such. Chickens had much more room to roam. They had lots of bugs and rodents to eat - whereas we kill all the bugs and rodents these days. Chickens got kitchen scraps and scraps from the family milk cow and the day's butter churnings. Chickens had plenty of manure from other animals to dig through as well as getting grain the other animals dropped from their feed bins. There were grain scraps left in the fields for the chickens to forage. Chickens chose their diet back then, their diet wasn't decided for them by a commercial feed company. Their diet was much more varied because more people raised their own food to feed their family - something relatively few people do now. Some would argue that with today's science, formulated manufactured feed is better. I don't know if I believe that or not since the advent of GMO seeds and research suggesting that today's grains/veggies have less nutrition in them than they once did, as a tradeoff for getting higher harvest yields with less seed.
We keep our chickens a lot differently today. I also don't think that farmers were worrying about trying to conform to an SOP. The biggest, strongest chicken in the barnyard was the one that survived to have offspring. It was the poultry fanciers that set down "the law" and bred chickens to get them all to look a certain way and to have certain qualities like egg and/or meat production. This is what I have been discussing with a friend - if the breeding for conformity to meet the SOP has been responsible (all or in part) for causing a decrease in the size of chickens.
I don't know that people would be able to keep a flock like they did 100 yrs or more ago. People are so far removed from where their food comes from, and they are always trying to stay young and fight death, that I am not sure that people (even me) are ready to completely throw their chickens outside to live as they once did, knowing that there will be a certain level of predation and death until only the strongest, biggest, and smartest chickens in their flock are alive and breeding. So in that case, in order to try to simulate the survival of the fittest cycle, separating males/females in order to get them bigger might be the only way that modern chicken keepers can get their birds to the size they should be, if they are going to keep their chickens in a more "managed" and "protected" manner like we generally prefer to do these days.
Definitely a thought provoking subject.