How many 6.5in driver equals one 8in driver

I've heard lots of speakers with 6.5 inch woofers. They can generate the lower frequencies but they always sound small to me. 8 inch is the smallest woofer size that sounds convincing. And both need a sub of 12 or more to move air below 40hz. 

I’m using an 8” “sub” in a secondary system. It’s more of a low frequency driver. Makes reasonable thump but a real subwoofer has to be 10 or 12 inches minimum.

Equal in displacement or sound quality?  

Just because a smallish woofer can theoretically put out a 30 hz note does not necessarily mean that it actually sound big & full in the bass in a speaker like a 12” or 15” driver in a bigger ported speaker can that falls off quickly after 35 hz . The smaller driver has to move faster & further than the big driver to produce low notes at higher volumes which can & does introduce distortion of various kinds as the voice coil heats up & changes impedance. 

You also have to account for xmax, enclosure design/ size, efficiency, etc. if you're looking for comparable performance between speakers.