The one on the left is a breed from 1957. The middle one is a 1978 breed. And the one on the right is a commercial 2005 breed called the Ross 308 broiler. They’re all the same age. And the modern breed is much, much, much larger.
When I was learning to cook, I’d follow recipes from my mom’s old cookbooks that she had when she was in college. One of my favorite dishes, steamed chicken with ginger and scallions, called for a three- to four-pound chicken. It totally screwed up my cooking times, and we ended up with many undercooked and overcooked chickens.