Fair enough if that's true, but question would still be why there's not even a theory?
If you asked me why more displacement makes more torque I could answer in one sentence. Torque is force applied to the crank a bigger cylinder allows more fuel and air to be burned in the combustion process applying more pressure to the piston which in turns applies more force to be multiplied by the crank through the rod.
None for added displacement and hp? Just got to go on cause we say so.