Larger diameter shafts allow more room to adjust necessary or preferred angle of fletching. Depending on differing factors from individual shooter to individual shooter, one shooter may obtain good flight and accuracy with straight fletch, another may require extreme helical. Factor in that if a shooter needs a 5" fletch to obtain good flight, even more room (shaft diameter) will be necessary.

Considering that fletching size and angle is crucial to good flight, and ample shaft diameter is necessary for certain rest designs, I personally will opt for a larger shaft area (diameter) to adjust fletching and obtain good rest clearance.

Also, the advertised and hyped penetration tests are usually based on the amount of penetration into a dry medium. An animal is made of lubricated tissue. When the broadhead blows open a hole ahead of the shaft, why would a small diameter shaft slide through any better than a larger diameter shaft?

Excerpt:

"I also believe we’ve been ‘hyped to death’ by some manufacturers about how to improve penetration. Their advertising has made it sound like there was a problem to be solved. But, for the most part, there isn’t."

"If you do hear a bowhunter talk of a penetration problem, it’s probably because the arrow didn’t go where it was supposed to. He either blew the shot, or took a risky one. And that’s what really bothers me about this whole penetration thing. Put the arrow where it counts and penetration isn’t going to be a factor. Put the arrow into a shoulder, and you’ve probably got trouble regardless of your setup."