It appears that his studies had to do with the relationship between shaft size and ferrule diameter. The thing about the small diameter shafts is, it is easy to find broadheads with larger ferrules. In fact, I don't think you can find otherwise.
Ashby had posted a preliminary chart in a thread over on Tradgang.com that showed average penetration for a variety of arrows using the same broadhead. I think it was GoldTips he'd weighted up to 600+ grains, to be equal to the larger diameter arrows he was using. Don't remember the brand for sure at this point. Anyway the carbon arrows did penetrate better. Not much better, only a couple of inches, but it was measurable and consistent.
I kinda drew my own conclusions from that. It may well be the shaft dia. vs the ferrule dia. is what makes the difference. I'm not sure Ashby himself has reached a firm conclusion yet. But it's obvious to me from his data that, using the same broadhead, a smaller diameter arrow shaft at the same weight with the same FOC will yield at least somewhat better penetration.
Doesn't mean I wouldn't still poke a hog with a 2315 or a 23/64ths cedar though. They've been doing great for me for a lot of years. It's just that I've still got a lot of crow left to eat over this shaft diameter thing.
