What ^^ said.

Basic principles of an engine is combustion. More air, more fuel = more power.

The cooler air is more dense, so per every cubic inch you are getting X amount more of oxygen. The computers will compensate for fuel. So with more air and more fuel being squeezed into the same amount room (more pressure) you also get a bigger bang.

The principle is the same as to why cars get less power in high altitude locations.