The big puffy ones of yours look like Cornish x - they are meatbirds, and bred to mature very, very fast and ready to process at a young age. If they aren't slaughtered for food then, they will outgrow their organs and die young anyway. They generally do not live long enough to produce eggs. There sole desire in life is to eat as much as possible. They rarely move from the feed dish. Some people leave food with them only 12 hours a day, and remove it for 12 hours- to somewhat curtail their explosive growth. It's sad but the market for them is high.