Yep. I have to admit I long held onto that point of view myself. But according to the arrow lethality studies on live game and dead animals that Dr. Ashby has done, smaller diameter arrows really do penetrate better, all else being equal.
Arthur, Ashby has written so much, it's difficult to remember everything, but I did find this that he wrote.
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Data was suggestive that shaft diameter did appear to be of significance in penetration. When velocity, broadhead and arrow mass were constant, shafts (1) smaller than the broadhead ferrule in diameter, (2) equal to the broadhead ferrule in diameter and (3) larger than the broadhead ferrule's diameter did appear to be a significant factor in penetration. No difference was discernable between shafts of varying amounts smaller than the broadhead's ferrule. If the smaller diameter shafts are assigned a factor of 1.0, then the apparent reduction in penetration was 10 percent (0.9) for shafts equaling the broadhead's ferrule diameter and 40 percent (0.6) for shafts of diameter exceeding than the broadhead's ferrule.
With a quick look, I couldn't find anything else that contradicted this. 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.