Bigger bore = more power. I agree, that's why we have big bore kits for motor bikes and atvs. Bigger bore and piston squeezes a larger mixture of air and fuel which then makes a bigger bang and pushes the piston and con rod down harder with more energy.
That only works if the induction system is able to simultaneously pass more air. And a big part of the increase is the larger Swept volume which increases the SCR.
What I mean is this, you can put a 5" piston on your pushmower, but good luck even starting it with that lil rope starter.. lol