To a point, higher voltage is more "efficient" than low voltage. Power is a product of voltage and amperage and to supply the same amount of power (or light), the amperage must go up a commensurate amount. What takes 1 amp at 120v takes 10 amps at 12v and so on. When the current goes up, so must the conductor size, also in a commensurate fashion. A wire rated for 10 amps will have about 10 times the effective cross section of a wire rated for 1 amp.

It's a little more involved comparing 12v DC to 120v AC when the discussion is power, but the principle applies AC to AC or DC to DC - lower voltage requires higher amperage for an equal amount of power (motor size, amount of light produced, heating element, etc)

TANSTAAFL.

Tom