Here’s a data point for you: we just butchered and sold a batch of New Hampshire broilers, 11-week-old cockerels. They ranged from 1.25 pounds to 1.5 pounds. These little broilers are what is meant by a “spring chicken.”
Hybrid broilers of the same age would have dressed out at 5-6 pounds.
This 4x difference in weight is why no one raises standard-breed chickens for meat anymore. The labor in raising chickens is per-chicken, but the payoff is per-pound, so it’s hard to make money with little chickens.