More is better for momentum and KE. In the case of diameter, less is better.

I hunt with the old pultruded arrows that are 15/64" (0.243") diameter and get great penetration on deer and targets. In fact I quit shooting them at targets since they either penetrate and ruin the vanes or are very hard to pull out. On the same target matt (stacked celotex), they will penetrate at least twice as far as my 2613's.

On a bag target the pultruded arrows will go all the way through. The 2613's sometimes bounce off if I have my bow turned down a little.

Like BobCo19-65 said, all else being equal, thinner arrow will penetrate much better than fatter arrows.

Allen