That's a great question. Now that you bring it up I suppose it could be a function of the prop. I don't know what the pitch of the propeller is but if I had to guess I would say the pitch is relatively high and that could be a big factor. Here's what Hobby Lobby says which makes me think you're on the right line of questioning
The big 4-bladed prop provides lots of thrust with only moderate speed
I have to work the next four days, but I promise to put a watt meter on the engine next Tuesday. Can't wait to hear everyone else's experience . . .