Just a reminder, wire size for both power and ground is determined by the breaker or fuse protecting that circuit, not by the expected load. 

In the event of a short circuit the wire must conduct high current until the breaker or fuse interrupts the circuit without overheating.

For an extreme example, consider if the load were a 10 mA LED pilot light connected to a 10 amp breaker circuit.  A short at the LED would result in (more than) 10 amps flowing through the wire.   While it is tempting to think we could use perhaps a 24 or 26 gauge wire, plenty good for 10 mA, that wire would glow red hot before evaporating with the possibility of causing other nearby wires’ insulation to melt and a cascade failure result.  Or an in-flight fire.  Hence the wire must hold up for the trip current long enough for the breaker/fuse to blow.