Greg,
I have no scientific proof to back it up other than my own experience but I truly believe it is the lateral cutting diameter that does the trick.......the reaching out and slicing as much tissue VS simply more blades and "cutting surface" that seems to drop an animal faster.

If you take a head with a two blade 2" cutting diameter (or more on impact) and compare it to say a 1" fixed head but for giggles throw 10 blades on that 1" diameter head and increase the "cutting surface" by whatever that math is?.........LOL you are still only damaging an area 1" laterally and affecting a smaller area.

Sort of like the absolute extreme you can think of (and I know it sounds rediculous but I think it has some merit in the "Diameter vs, Surface" arguments)
Take a broadhead with a 24"two blade cutting diameter (yes I said 2 FOOT cutting diameter. ) and hit a deer with the blades straight up and down with enough energy to pass through...........you've effectively cut the poor sucker in half and the track job will for all intents and purposes be a relatively short one.
Now take a broadhead with a 1 1/2 cutting diameter and throw enough blades on that puppy to equal 24" worth of "Cutting surface", just stack as many in there as your heart desires..........

Which one caused more damage?