Makes sense for field point tipped arrows but what about broadheads when the shaft is following through a 1"+ hole lubricated by body fluids? I don't buy it.
I used to not buy into that either, and for the same reason. But Dr. Ashby's arrow lethality studies actually do show increased penetration for arrow shaft diameters that are smaller than broadhead ferrule diameters. I had to eat a little crow over that one.
His work does NOT show enough additional penetration to offset 100 grains difference in weight however.