Long answer: Different planes have different ratings in the eyes of passengers. You 20-passenger plane might be rated lower than your 30-passenger plane and therefore get fewer passengers. The reverse might also be true: a smaller plane might be rated better than a big one. But this won’t help much in this case since all 20 seats will be full anyways. Assuming the two planes are rated equally, you should get a 100% load with the smaller plane.
If nothing else changes then in theory yes. However there’s a good chance if you change an aircraft you’ll change something else even if it’s unintentional (flight attendant to pax ratio, aircraft rating etc.). Your best bet is just some trial and error.