Charles,
It basically is materials science with some aerodynamics added in.

I believe that an efficient prop is a thin prop--but I could be wrong here. But lets say I'm right. So the thinner you can make the prop, the easier it is for the motor/engine to turn the prop. So that is my version of the aerodynamics argument--experts are welcome to chime in here.

However the prop has to hang together. In a glow engine, the power pulses come either every cycle (2 stroking) or every other cycle (4 stroking), so between the pulses, the prop is rotationally slowing down-- a lighter prop for example would slow down more than a heavier prop. That acceleration puts a lot of stress on the blade, so the manufacturers have to increase the blade thickness.

With our electric motors, we are getting ~10-14 (depending on motor type) impulse like pulses per revolution (talking about commutation) so the torque is smoothed out over the entire 360 degrees. That means the prop tends to be running at a more constant revoultion (rpm) level, and isn't being whipsawed around as much as in the glow case. The manufacturer can thin the props down considerably. Most "E" props are noticeably thinner than the glow counterpart. I think that's why they are more efficient (but I have never actually checked this out in a flying model!).

Another way to make a prop thinner is to use stronger materials, which is why I think carbon fiber props are so popular at the high competion levels. It allows the glow engines to turn a larger diameter prop than if you would use a thicker prop. The large diameter prop is what gives you the thrust when the airspeed begins to drop--thrust as a function of airspeed is a steeper falling curve (at a nominal fixed rpm--which is what all the fuss is about using the 75 size glow engines now).