There is no substitute for cubic inch or in this case surface area

Advancements in woofer and cabinet technology no longer require large diaphragm elements to produce accurate bass response capable of properly loading a room and producing the benefit of enhancing soundstage you seek.  The effect on soundstage is related to the accuracy of the low and mid bass, and the woofer’s ability to move enough air in correlation to room size to create  the effect.   For example, my speakers are known for the fast, articulate, and dynamic bass response with two vent loaded 5.5” woofers.  These woofers are a rigid, long throw diaphragm design that move large volumes of air even though they are small diameter drivers.  25Hz test tones are audible (a little over 3db down); however, there is a steep roll off after that and 20Hz test tones are not audible to my old ears.  This level of response is amazing from such small drivers.  In fact, multiple smaller woofer speaker designs have the advantage of wider dispersion and less beaming than older large diaphragm woofers, enhancing the soundstage better. Therefore, it is not necessary to have a large diaphragm woofer to get deep, accurate response that enhances soundstage.  

The technological advances include, but are not limited to:

1. New magnetic materials with higher field strength (higher Tesla rating). 
2. New magnetic designs (for example, my speakers use a ring magnet structure) that are more accurate and more efficient transducers of electrical to mechanical energy.
3. Long throw armature designs moving as much air as a larger diaphragm, short throw design. 
4. More rigid diagram materials and computer assisted design with less break-up effects resulting in lower distortion. 

These advances permit the design of small size woofers that have the advantages of lower distortion, speed, dispersion and accuracy while moving significant quantities of air compared to older large diaphragm designs.  Within a given brand, woofer size increases with larger, more expensive models that are usually designed for more voluminous rooms in order to correlate the volume of air moved by the speaker with room volume.  

I just experienced exactly that. My speakers are 12" 3 way. I've been looking for a smaller speaker that can best them, and have come up empty. 

Most recently, I auditioned a pair of ATC SCM20P. Having heard the 50s and being told that all ATC speakers share the same sound, I had great expectations. And they are great sounding speakers but just don't have the scale of sound I'm used to.

Since my existing speakers are JBL, I also auditioned a pair of 4309s, the 6.5 inch version of the 4349 which is a great speaker. And they too, are great sounding speakers overall but just sound small. 

The ATC SCM50s only have 9 inch woofers but sound as large as mine so maybe there's just a threshold of size for that big sound. The wavefront a larger woofer puts out is recognized by our hearing as representing a bigger scale of sound. 

That's the great thing about extolling the virtues of home-built; nobody else has a means for comparison to confirm or refute the claims.

The term for the combined cone area and how far they move is called SD.

Also in consideration if how long the voice coil is compared to how far they can move and being under control properly.

Multiple small drivers can do what any big driver can do as well, if used correctly can even be used to cancel out issues many particular drivers tend to have.

I like the sound of big drivers but also the sound of small ones designed and used correctly, both can be very satisfying.

I have had horn systems that would blow away most all big driver systems in dynamics and incredible overall live sounding output and they used very small drivers.

I was also designing a very high end and complex system for a car project with massive horns built into the vehicle and dual 21" infinite baffle subs before I decided to go full time RVing so I sold that project off.

There is simply no one solution for what can be archeived for any particular size of drivers. Then of course first and foremost the space it will occupy and room treatments being top priority, always.

My next DIY speakers are going to be Frugel-horn Joan with Mark Audio MA200 eight inch full range drivers just because I have not done this before and it should work really well in our full time RV where my Egarhorns are the wrong design and far to big, line arrays I have might work, wanted to go with OB but not a good place for it....I could end up building something else but I like the idea of no crossovers and my little Baby Sophia 10w PP amp, fully upgraded of course.

Rick