I never really thought about the affect of the outside temperature on the power of the engine, but it makes sense when you think about cold air intakes. Does this mean that in winter cars should me noticeably more powerful than in summer? And what about fuel efficiency? I'd think if cold air is better then MPG should go up in winter at least once the engine is up to temperature, but I've heard cars are supposed to use more fuel in winter, I never really noticed myself.
The reason MPG goes down in winter is largely because the engine takes longer to warm up, and thus you drive longer with auto-choke enabled.
Furthermore you usually have the heater all the way up, which further prolongs the engine warm up (the heater "steals" the heat from the engine), and finally heated windows use a lot of power, so the generator has to work harder, which in turn also costs fuel.