A couple of basic things you should alos understand are that...

Fuel... Has a potential chemical energy, a normal Piston engine will turn about 12% of that potential energy into power to drive a car etc... The rest is wasted in heat, sound, and other types of energy...

I am not sure what the percentage for a glow engine would be, but it is porbably much worse than 12%

So when an engine becomes more powerful it is either...

1. Burning fuel more efficiently, and so extracting more than 12% of the potentil energy in the fuel...

or

2. Burning more fuel, and thus producing more power, in a single stroke.

That's a very basic look at the physics behind buring fuel for energy but it should give you a good idea of why you are now using more fuel...

Also a Chev engine probably only extracts about 3% of the potential energy in the fuel it burns... Sorry could not resist...

Matt