I basicly have found the same thing. I have shot "skinny" carbons, the ICS carbons and all sorts of differnt aluminums. The smaller diameter of the shaft the more it penetrates in most materials. I also think that the stiffer nature of carbon puts more of the impact energy in line with the broadhead which will increase penetration. On the other hand, I have found that the larger the diameter of the shaft, the better the accuracy with broadheads. This of course is provided both arrows are spinned and tuned properly to the bow.

TAKE YOUR KIDS HUNTING AND YOU WON'T BE HUNTING FOR YOUR KIDS