Alright...This is what I think.

If the engine is bored out more then its stock form, the circumference of the piston sleeve changes. Since there is a larger area, the more fuel/air mixture is needed to create a even combustion cycle. If it uses the same amount of fuel/air mixture as the original block size, then it would actually create less power since the ignited fuel/air mixture would take up less cubic centimeters so there for, there wouldn't be a sufficient amount of power. Now when you add more fuel/air to take the place of the extra cc's of a bored out engine, the ignited mixture would take up the extra cc's causing more torque.

I just think the above and cannot fully back up my theory but I hope this sparks some ideas.