Airflow is what dictates maximum potential horsepower. You could bore/stroke a flathead out to 500ci, and it wouldn't turn over 1200 rpm before it was maxed out in the breathing department. Only thing it would be good for is a tractor. Horsepower requires airflow. Once the airflow path is maxed out, it's simply maxed out.
We'll ignore the 500ci reference, as it was only to make a point. When an engine has limited airflow, adding more displacement doesn't make that much more peak horsepower. It's main effect is lowering the rpm where peak horsepower is made. It also tends to add torque.
If you have an airflow limited engine making 200 hp at 5000 rpm, and then add stroke... It will likely still make 200 hp, but do so at let's say 4500 rpm, while adding more average power below that peak.
