18s vs 15s

im lookinv to see if theres is that much of a difference in air movement for 18s compared to 15s if it has the same motor force, same power, same xmax. and ive heard they are slightly less punchy? does that mean it wont hit as hard or what does that even mean lmao. im really just trying to get as low and loud as possible and move as much air as i can, i dont rlly care about sound quality or the reponsivness of the bass. i know the 18s more fit what i want but i wanna know if theres THAT much of a difference between those 2 sizes. if it matters im specifically talking about the fi hc-15 vs the hc-18 and either way id be tuning to 28-29 hz in a 5 ft box and putting it in the back seat of my 05 accord