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

Bigger definitely will open it up more. Although there is much to be said for stroke, frequency, and duration. Especially duration. The longer the session the more it opens things up and the less constrained they get.

There is no replacement for displacement. Axiomatic in the car world, and as well should be for producing bass.

Of course, on both fronts there ARE replacements for displacement. But slapping on a turbo or just increasing xmax isn't a true replacement Hence the non-linear price increases for well executed power at smaller displacements.

Much value being offered herein albeit cryptic. 

I believe it's largely fashion.

Not everyone wants to, or is even able to accommodate speakers with huge 15 inch bass units like me. 

Drive unit size is certainly a contentious subject with various arguments for and against large and small drivers.

It's certainly not a question of bass content either as some small units can dig lower than some larger ones. Depending upon factors such design (greater number of long throw units) and cabinet loading.

However there do seem to be differences, size does matter.

Permit me a gross generalisation if you will, but I'd say that smaller units sound noticeably quicker with transients.

On the other hand, larger units tend to have a far greater sense of ease as they go about their work.

The 15 inch Dual Concentrics on my Tannoys are easier on the ear than the slimline Revolution 3s that they replaced, if perhaps surprisingly a little lighter in the bass.

If I had to chose, and if I wanted a more relaxing sound, I'd take the second option.