Size and shape only matters in the fact that size and shape can often be the determining factor in having 2 holes, sharpness being equal. Which size and shape is best takes testing and time to form accurate conclusions, or you can just trust someone who has done the testing already...that would be me.
To say I have killed a ton of animals would be an understatement. The fact is I have arrowed right around 15 tons of 4 legged game and 2 tons of turkeys. The knowledge learned by taking so much game was multiplied by the fact that I have used roughly 100 different designs and styles of heads in the process. Add the fact that I get pretty technical in the observation of damage done by individual hits and you have a lot of experience.
One of the things I have learned through all this is there is little parrellel between foam testing and animal testing. The separation between a good and poor penetrating head is magnified much more in game than in foam. In game the initial penetration of hide is the key to pen and as a whole, and certain heads do a much better job
It's probably no surprise that cut on contact heads penetrate better, but few have tapped into the mystery of concave shape broadhead structure. It is a simple explanation why these heads perform so well. You see when broadheads first make contact with hide, the most resistance occurs between contact and when the head pops through. There is some hide stretching before the head pops and often the blades are fully contacting hide befo and the head pops through. The short heads that are so popular now are more prone the this and the blades are having to press through hide rather than slice. With the keener concave design there is less pressing of the blades and they pop through with less resistance. As with heads like the Simmons less than 1/4 inch of blade presses before popping through, and once pen to is established the resistance is minimal from that point on.
