basically it comes down to weight. gas burns hotter so it needs bigger cooling fins which = more weight for the engine. gas also requires a ignition system which weights more than half a pound w/ the battery. gas also is less powerful than methanol so you need a bigger gas engine to produce the same power as a smaller glow engine which = more weight

gasoline really isnt totally efficent until around 50cc area, smaller gassers will feel the weight and lack of power (in general). though with new smaller gas engines dle30cc ect they are getting lighter and more powerful