It's a classic tradeoff between efficiency & bass-extension vs. size. All else being equal, the bigger the size of driver and cabinet, the more efficiency and extension it can yield. There are ways to tweak this, e.g. controlled directivity via horns/waveguides (which can be a very good idea), but I'm definitely in the camp that bigger is better. Modern materials and computer design have made large strides towards our ability to build better stuff at a given size, but if you want it all then bigger is still better, and you still won't hit diminishing returns until you're at some pretty
big honkin' speakers.
Sure, a small driver can produce a sound at 20 Hz, but its SPL level at that frequency will be several orders of magnitude below usefulness. The speaker manufacturer can damp down the mids & treble to yield a flat response with "great" extension on a small woofer, but then the overall efficiency is terrible. Sometimes you'll see small speakers quote seemingly impressive bass extension; then just look at the horrifically low efficiency that goes with that number (
if they're honest). Your inclination might be to throw gobs of power at the speaker to rectify the situation, but guess what else small speakers tend to suck at? Dissipating heat from a! ll that power -- more than 99% of the power delivered form your amps is burned up as heat in the speaker's voice coils, even in
efficient speakers (more like > 99.99% in inefficient speakers). A small hifi speaker that can handle 1000 Watts is rare indeed.
With rooms there is a "room gain" that affects the lower frequencies. The smaller the room, the more the
lower frequencies are reinforced by the room. A large speaker that measures flat anechoic (i.e. effectively open space; ignoring room effects as much as possible) is going to be bass-heavy when placed in a small room. The way a speaker's design accounts for room gain varies between manufacturers. For this reason, a big speaker designed to play flat into a BIG room is going to sound bass-heavy in a small room. Then -- eureka! It's
too big for that room
You can much more reasonably "get away with" smaller speakers in a small room, especially in a near-field configuration -- but in those cases watch out for issues with coherence if the tweeter/midrange drivers aren't very close together.