It takes roughly 1/3 to 1/2 more displacement in a gasoline powered engine to develop the same horsepower as the glow engine. So if the glow engine was 1.8 cubic inches (30 cc) then a gasoline engine of 40 to 45 cc would be very close in power.

The fuel energy is part of the reason, apart from the differences in heat content. It is all about the mass of the material that is being heated up and expanded. Running an engine on gasoline, there is a much lower mass of gas and air going through the engine to do work. With methanol, a much larger mass of fuel and air is doing the work. Partly, this is also why higher nitro fuel makes more power than low nitro fuel in glow engines as well. Nitro weighs more than methanol, and you can run a much higher mixture of fuel to air ratio.