Cornish Cross in particular are fantastic at converting feed into weight gain. In 10 weeks, you can go from a fluffy little ping pong ball sized chick to an almost 10lb live-weight bird. It's often unwise to raise them past that age, because the rate of weight gain puts so much stress on their internals that they'll frequently keel over from a heart condition. By the 8 to 9 week stage, they're already so heavy that they can only take a handful of steps because having to plop down and have a rest for a bit.