No, size absolutely has much to do with how easy a subwoofer can slam lows. That said i whole heartedly acknowledge that there are 10's and 12's that can slam lows just as well as most 15's and 18's. But for a 15 it is easier and shakes harder and thats just how more cone area on a sub
Theres a proper way of doing things and theres the way that gets you minimal results. Dont be surprised when you roll up to a guy that actually knows what hes doing with ported boxes and his 8s out slam the lows vs your 15. Ported is the way to go if you want strong lows.