I think 1/4 scale is the breaking point for gas vs glow. It of course depends on the type of plane and how much power is desired. A 1/4 scale cub might fly adequately and efficiently on a 1.50 glow, but a 1.50 will not give a 16lb Extra 300 unlimited vertical. In smaller aircraft below 1/4 scale, gas engines weigh too much for the amount of power they produce.

I fly only 1/4 scale planes, and I just switched entirely to gas.....for convenience (less equipment to carry) and economical flying. I was paying $3.00 per flight with glow....now I can fly for 2 weeks on $3 worth of gas. Gotta love it....and no cleanup.
PT