Simple way to explane electricity, its like water in a hose, the pressure of the water in the hose is the voltage, the flow is current in watts/amps or whatever.
volts allows electricity to force its way through a restriction (ohms) and amps is how much electricity can flow.
Heat is a direct relationship to amps, HOWEVER when talking about a motor you have to factor in EFFICIENTCY, the ability to do work with the amount of electricity (or heat) is applied. so figure a high efficientcy motor is 97%. so 97% of the energy applied is work (the motor spins) and the remaining 3% is heat, friction and sound. anytime you restrict a motor it makes the voltage drop and draw (amps) go right up. a special effect of a electric motor is that a spinning motor creates COUNTER VOLTAGE... countervoltage reduces the amount of volts/draw is required to make it do work since in a way its powering itself- when you add friction to this motor it HEATS UP. why? remember efficiency??? it was 97% running free with no load, now figure its at 50% so now half of all that electricty you put in is turning to heat!
this is why high KV motors more often than not heat up more compaired to low KV motors. high KV motors spin much faster and generate more friction both in the drive train but also the wind and the terrain, this slows this motor down driving the effiecientcy down making the amount of heat generated higher.
on topic i found out something... the outrunner i used on my rc18 is 600watts... a mild/hot 4700kv motor for a 1/10 is around 440.... i LOVE outrunners, so much power in a little package, its going into my digger once i get it!