My understanding is that with a bigger driver it's harder to minimize cone resonance in the frequencies your trying to produce. I.E. It's hard to come up with a material which is light, stiff enough to move a lot of air without resonating, and also have a large surface.
As far as the write-up on sound Intensity goes ...I only gave it a two minute glance ... since the particle velocity vector concept didn't jump out at me ... I'll have to look at it later.
My current thought is this has something to do with having two different drivers move a similar volume of air. A 12" cone will need to move less distance then a 8" one. How that relates to the equation presented needs a more awake brain then I have at the moment.
