This is a cornish cross chick on the right, and a buff orpington chick on the left. They're exactly the same age. At this point the cornish cross weighs about 3x what the buff orpington does.
at 2 weeks of age:My guess is that the cornish cross eats about the same amount of feed per pound of chicken produced as the heritage breed does, but that it does it in a shorter amount of time. At this point we're on feed restriction for the cornish cross.
Feed restriction prevents the cornish from eating themselves to death. If given all of the food that they want they'll completely stuff themselves and die, probably of flip.
Flip isn't a problem with heritage breeds who grow much more slowly and seem to regulate their feed intake better than the cornish cross. For me this is one big advantage with the heritage breeds. You put them out on pasture, make sure they have food and water, and walk away. They'll happily grow and forage without any work on your part for the next 6 months.
The cornish aren't nearly that self-sufficient. Part of the reason that people raise them in pens that you move is that it's easier than cleaning a coop of manure, and it encourages the cornish cross to forage. Otherwise they'd pretty much stay glued to their feeders.
There are various huge chicken producers that produce "free range" chickens. It would be more accurate to say they're "barn raised". One producer simply cut doors into the side of their broiler barns and during the last 2 weeks of the chickens life opened the door. Of course, since chickens are territorial, and broilers in particular want to stay close to the food and water, it's not unusual to find that not one chicken ever goes outside. In a rather cynical move, some of the producers have little lawns they maintain between the barns so that they can claim to be "pasturing" the chickens. None of the chickens actually get on the grass, but theoretically they could.
For my chickens they're out there on the ground, pecking and scratching and doing what chickens are meant to do.