Hmm..I'm not sure if I get that analogy.
but a quick rundown:
Amps x Volts = Wattage.
Car systems are set to a static 12V so amperage is the only thing that changes can be amperage to meet the wattage requirements.
Best analogy is plumbing pipes
Voltage = Water Pressure
Amperage = Water Flow Rate
Water flow will only use as much as is required. ie. Turn signal wants 1A, the amperage will only pull 1A.
Only thing is to ensure the water pipe (Resistance) is rated for the max water flow (amperage). Since we are going from 120A to 150A, I doubt the power wires and ground straps (water pipe size) will have any issues with the minor increase.