I think you are confusing bullet weight with bullet design. A heavier bullet in the same caliber and design will give you better momentum for increased penitration and a better ballistics coeficient because it will be longer with the same diameter.

For the same bullet design in order to do more damage you would have to drastically increase velocity, which really doesn't happen with the weights you are talking about.

A lighter more explosive bullet design may do more damage to the vitals with a well placed shot, but the heavier bullet with more controlled expansion will have better penitration with a less than optimal shot if it has to go thru heavy bone or a lot of extra tissue (frontal or severe quartering shot). Or on very large tough game.

Knock down power means nothing in hunting ballistics. It has to do with the wound channel and how much the bullet can damage as it goes thru the animal. Nothing will be absolute as far as knocking a deer down on the spot, especially if you shoot it in the vitals. Maybe if you shoot it in the shoulder and can smash both shoulders and keep it from running. And for that I would go for a heaveir bullet that would penitrate better.

I would rather have the best of both worlds. A bullet that penitrates well and will exit the animal and expand well enough to create a large wound channel and do plenty of tissue damage to the vitals.

Basically a heavier bullet is a bit more forgiving and may off a bit better accuracy at longer ranges. Really depends on your rifle and how you hunt.

Paul