The power and torque needed to turn a prop is exponential to rpm. The torque needed to turn a prop is proportional to the square of the rpm and the power needed to turn a prop is proportional to the cube of the rpm. Thus, to double the rpm of a prop, you need to quadruple the torque delivered to it. Doubling the rpm of a prop also quadruples the thrust BTW. Four times the torque at twice the rpm is eight times the power.
A .15 glow engine will spin a 8x4 about 13,000 to 14,000 rpm. That exact same prop will turn about 7,000 to 8,000 rpm when bolted onto a Cox .049 Texaco glow engine!! An engine with only about 1/3 the displacement.

Not understanding the exponentional nature of prop load has lead to the misconception that 4-strokes have more torque than 2-stroke engines. It takes a lot more torque to turn a 10x6 14,000 rpm than it does to turn a 12x6 8,000 rpm.

I do a lot of dealing with electric motors and believe me, fans smoke more electric motors than nearly any other application because someone decided to change pulley sizes without understanding the exponentional nature of the load.