How to determine the correct arrow type?

[Rustywreck]

We have to be sure we are talking about the same thing in regard to thickness. Do you mean shaft diameter, or wall thickness?
The wall thickness will add weight to the arrow, which will help penetration.
As far as the diameter of the arrow effecting penetration, I agree with Varmint Hunter - if it makes a difference I don't believe it would be enough to be concerned about. Once the tissue of the animal is cut, the resistance is pretty slight. It's pushing the broadhead through that takes energy. Foam targets don't make for a fair comparison to real game.