Substantial Power Gain from 13cc Displacement Increase - Why?

Why is it that if you over bore some 4 stroke engines the performance gained is more than what you would expect with the minor increase in actual displacement. As an example a YZ250F with a 2mm larger piston gains only 13cc in displacement yet will likely gain a few horsepower throughout the rev range with no other modifications. The gain actually comes from the larger bore"unshrouding" the valves in the cylinder head allowing for increased air flow. The 5 valve head is especially bad due to the crowding. All the power from a 4 stroke is in the head.