Glow engines are light and the power output compares favorably with spark ignited gasoline engines of similar displacement. Another way to phrase that is, they have high power density, or a relatively high power-to-weight ratio.

In the past, very small spark-ignited gasoline engine had to carry around a coil and some other baggage, just to create the spark. Glow fuel and glow plugs eliminated that weight, immediately improving the power-to-weight ratio.

All in response to the last part of your question.
Best wishes,
Dave Olson