Just how big is that driver?

That is why women can't park their cars because we tell them 6 inches is really 12!

The sub I have is sold as 12", which is the size of the mounting hole, so the size of the driver is a bit smaller.

I'm 5'10" but think I'm taller!

I am serious. IMHO, equipment should as much accurate specs as possible. This should help consumers narrow down the compatibility options.

Is this a 'never mind the quality, feel the width' sort of question?

I have noticed the JM lab drivers are fairly accurate if you account tor the surround. If you add the frame then it doesn't work out so neatly.
The hole in the cabinet may only accomodate the basket spider and magnet assemblies with the whole shebang held securely in place by virtue of a mounting ring which must rest on the baffle. Since this is very variable from one manufacturer to another I would not consider it a meaningful measure.
It is generally not helpful at all to concern oneself with the size of the driver. I do not know pro applications and it may be relevent in big spaces. My 15 inch woofered speakers do not have the bass that my double 8 inch speakers does so again it boils down to a great many parameters working in concert. Uless you want to argue that 2 x 8 = 16" but in all seriousness the reactivity the resistance to movement as well as piston speed = magnet and voice coils along with cabinet geometry all make a big difference.
Its not important when considering a bass transducer. My current drivers are speced to a low of 30 cycles but I lived quite happily with others that made no claim below 50 and were falling fast below 80. Because that is where the music- my music - anyway lives. Save the rumble for HT please.