[BrianG]

To move any vehicle, you need power. The heavier the vehicle or the faster/more powerful performance you want, the more power you need. Power is volts times current. Let's say you need a maximum of 1500 watts to get the performance you want for the revo. At 11.1v (3s) that would be 135 amps. However, if you increase the voltage to 4s (14.4v), you need 104 amps.

There is a bit more to it than that, like selecting the right motor and gearing, but you get the idea.