Even a light arrow will penetrate the body cavity and allow an arrow to do what it has been doing for thousands of years. Any pointed projectile for that matter is designed that way. The heavier arrow does not kill faster or make the animal more dead. That's rediculous to say and I'm sure even the heavy arrow advocates will agree with me on that
The heavier arrow has a better chance of getting into the body cavity in the first place, especially in bad situations. That's the whole point.