Small drivers vs big drivers

The question is a bit broad and could cover a large response with respect to the advantage and disadvantage of large vs. small driver. 

First I don't think the OP was interested in line array.  It's a different class of speakers all together.  I think he was referring to for example having 2x7in drivers vs. one large 10in driver.  I don't think the reason of using line array is because one needs that many drivers to replace a single 15in. driver.  Line array is not meant to compensate for low end response.

In general a large driver will have more bass response but lacking the transient speed of a smaller driver.  So what you would do is having two smaller drivers to get the same bass response of a larger driver, but retain the transient of a smaller driver.  Also in general, a small mid range driver (2in or 3in) will have better midrange transient speed compared to a 5.5in or 6.5in driver, but the larger midrange driver will have more weight in the lower midrange especially on the male voice reproduction.

Large drivers also have the benefit lower distortion since it does not have to move a lot to generate the same amount of pressure vs. a smaller driver but of course you need to mix and match driver size since there is no single driver that can play the entire freq. range.   

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.