If your ESC wires were smaller than your motor wires, then you should be suspicious that the specs are mismatched. Fact is you could solder the 12A motor leads to a 75A ESC, basically if the ESC is bigger than the motor needs and the wires are bigger.. then you're laughing and the ESC certainly won't be under any stress. It might weigh more than it has to is about the biggest drawback.
ESC's don't force current upon the motor, they supply what it draws based on the rpm and load. So a big ESC doesn't do anything to a small motor other than perhaps offering less internal resistance and a couple more rpm.