Need help understanding external power on Arduino
I think you may not fully understand how the servo works. Most servos have a power, ground, and PWM line. The PWM line will pull very little power. This line is what the microcontroller will actually be controlling. The Power and Ground lines are where the servo actually pulls its power from.

The power line can be connected to anything as long as the ground lines are at the same potential as the Arduino. If you are using an Arduino board, the voltage regulator that is on the board is limiting your power to the servo and not the microcontroller itself. If you are using an Arduino in an IC form (ie: breadboard) then you will need to make sure what ever you are using as your voltage source (power supply, battery, voltage regulator...) is at the right Voltage level and can handle the current of the servo.