Interesting. I just measured a driver in one of my PJB cabs. The cone (from the join with the suspension roll surround) is 3.75" in diameter, which gives a surface area of 11.05 sq in. Multiply by 4 and the cone surface area of a 4x5 cab is 44.2 sq in.
A 1x10 with an 8.5" diameter cone would have a surface area of 56.75 sq in and a 1x10 with an 8" diameter cone would have a surface area of 50.28 sq in.
In practice, because cones are not flat, their surface areas will be larger. So there is a difference, but it isn't enormous, especially in the case of a 1x10 with an 8" diameter cone.
Edited to add: there is a coupling effect from multiple drivers in a single cab, which is also relevant.
Area is not the whole story, of course. You need to factor in excursion, plus how the cone behaves as it moves. A smaller cone is likely to be more rigid and flex less, which will increase its ability to move air cleanly. Having compared my PJB cabs with various 1x10s, I can only say that they are not left wanting in either volume or bass weight.
One man's meat is another's poison, of course. They do the job for me, which is all I'm bothered about.