Bass Driver Size - how much better is an extra inch?

I went from two way monitors with a 6" woofer to two way monitors with a 8" woofer. Not only is there more bass but the quality of the bass changed. It seems more tuneful and resonant for lack of a better description.

I use a pair of 18" drivers in a 14 x 27 x 8 1/2 ft room.  No complaints.  An extra inch would basically take the drivers up to 20" or 21".  Not sure that would make much difference.

It is an engineering compromise, just like almost everything is in life. It is basic physics.

The larger cones move more air and hence provide a much more realistic and voluminous bass. However, the larger the cone gets, the less rigid it becomes, affecting the sound negatively.

If you could find an infinitely rigid material, which would never lose its shape regardless of how much air pressure it was exposed to, then you could build infinitely large speakers. But real life has limits :-)

There is also one more very important factor as to why most drivers (and hence the speakers) are smaller these days.  It is the WAF :-)

Makes sense very large drivers loss some ability to handle detail.  So there is a sweet spot between, say, 7 to 12 inches. And beyond 12, you would be in a subwoofer zone where detail is less important and less feasible.  
Personally, 10 inches on the b&w 800 seems ideal.  
There is only so much tech research can do while trying to cruelly shrink a bass driver and maintain sq.  Physics has its limits
Manufacturers and designers need to wake up and not artificially  suppress quality bass sound to accommodate a smaller cabinet.  How cheap are things getting.  Remember how they trimmed the size of a carton of oj a few yrs ago.

The wharfedale 225's as reviewed by three different reviewers in stereophile, who all agreed that the speaker has phenomenal bass, uses only a 6.5 inch bass driver, ported at the bottom surface of cabinet. One reviewer, I think the late Art Dudley, was taken back by how much useable response the speaker had at 31.5 hz! It was confirmed with measurements. He said he heard what he heard....just one example. I still own a pair and still love them, but My tannoys with 10 inch driver somehow seem effortless, maybe more relaxed.