Small drivers vs big drivers

A lot of general information here.
first, you can make a small driver go below 20hz. We did it several times back in my old Marcof Electronic Days. We started with a 91db 8 inch peerless driver, when we were finished, this driver had an F3 of 19hz in a sealed box, what isn’t talked about much is trade offs. When we were done, this 8 inch driver had a sensitivity of 82, mass was high and the spider and voice coil former didn’t appreciate it under real power, but it certainly did it.
Next the early Totem Forest used a HiVi 6.8 inch driver that had high excursion limits and used 3 inch voice coils and a vented voice coil, that woofer did a good job with thermal issues.
Next, large drivers have no problem with speed as long as they are properly designed with enough motor to match the mass.
Next, the big deal between a large driver vs a small in bass output is just the amount of air it moves. I believe that it was Erik that mentioned above that it takes 2 8’s to move the air of a 10. It takes multiple drivers to move the air of a 15 inch woofer. Its that air movement that gives you that bass impact, not necessarily the lowest frequencies.
The higher the sensitivity with drivers, the harder it is to get LOW frequencies. That is why very high sensitivity pro drivers don’t go down well. It is possible to get low bass and good sensitivity, but it takes some engineering in the drivers. (Legacy comes to mind)
I hope this all helps,
Tim

Driver size has nothing to do with the frequency it can play. 20hz is simply the driver moving back and forth 20 times per second. 

Size does matter how far the drive has to travel to make the sound loud enough to hear. Pressure being a function of force x area (force being mass x acceleration etc). 

So a large driver needs to travel a lot less then a small driver for the same Sound Presure Level. 

Large drivers have trouble playing high frequencies do to not bring stuff enough to move 20,000hz without flopping around like a flag in the wind. Not to mention they will beam due to the sound wave length being shorter than the surface area creating it. 

Look at headphones. They have great bass from a tiny driver. It however only has to play that bass very quietly due the sound not needing to trave very far (the “room” is also tiny)

when small drivers travel far there is a lot of bending force on them from the driver surround. The voice coil also needs to travel farther and stay aligned. Then there is the electrical issue of long travel but that if for another day. 

Many good points. All tech stuff aside, I had a pair of jbl 250ti's for many years. They were a 4 way speaker with 14" woofers. I sold they recently and bought a pair of Revel Salon 2's with 3, 8" woofers.The Revels are  better speakers in every way except dynamics and base impact which is not a small feature. If you like the live sound of rock, the jbl's were hard to beat. Also, with the sensitivity the jbl's they played very well at low volumes. I did miss the bass impact of the jbl's, so I added a JL sub. I do love my current system, but the was cost was crazy. I may have been able to make a better choice in speakers, got the best of both worlds and kept my cost down (perhaps Legacy). But higher end speaker comparison is difficult because of the limited distribution.     

I am fortunate to have extensive experience with both, but every ~ multi ~ 4" driver speakers I’ve ever heard needed subs. However, I had a 7ft pair with ~ 8 x 6.5s which went plenty deep

I currently own Emerald Physics KCIIs which has a 10" concentric driver plus 2 regular drivers, but their 2.8 series has 2 @ 15" carbon fiber speakers, one being concentric ~ $9K a pair. I hope to have a pair one day soon