The argument for smaller drivers delivering more punch is just not true.
A well designed 15" will be better than an equally designed 12". A larger cone area can be lighter, therefore more efficient, having less distortion and the ability to move more air. Smaller drivers must be heavier to have the same amount of punch or low-end reach of a larger driver.
Find yourself a really well designed 18" woofer and you won't ever say a 6,8,12, etc. has more punch.