I observed the rebuilding of a 12KW, 120V gas-powered generator. I was amazed at how small the generator portion was (roughly the size of an electric motor you might find in a major home appliance). The windings were pretty thin magnet wire, like you would find in an appliance motor.
Getting the 100A generator output to the breaker box requires heavy-duty wire. That same current originates in the generator windings. How is the thin wire in the windings able to carry 100A (which far exceeds what wire that size is rated for)?