What makes speaker's sound big?

Accurate frequency response AND coherent phase from 17hz to 30khz and listen at reference volume. 

Result: BIG SOUND. 

One does not need large speakers to achieve big sound and throw an immense, deep soundstage that goes on forever. But the room size matters, distance to the speakers, room treatments as well as the specific drivers for spl potential. That's why to get the above, larger speakers are often needed for large rooms. 

Can you ditch the DDs and get that in your room? If your room is fairly lively, undamped and you can sit close, yep. Obviously, not without subs and expensive speakers. 

This thread keeps going...So here's another thought, not specific to the speakers the OP mentions. 

(Again, the real answer is undistorted, real full range loudspeakers/subs. Size does not matter, but actually achieving the above sentence IS certainly easier with physically larger speakers. At least at lower cost.)

However, nobody SHOULD want big sounding speakers. I'll even go further. No one should want speakers to CREATE scale and sound big. A speaker reproducing stereo (almost by definition) should simply disappear!

Yes, room reflections, speaker style, on and on are important, but none exclusively provide or control auditory image-size and scale. 

Let's back up. What is the recording? A piccolo solo? A floor to ceiling piccolo would be undesirable, even comical. A bumble bee buzzing around? Small, precise image flying around.

Two of the DIRECT causes of image-size and scale are 1) Volume and 2) the Recording itself. 

If your speakers are making, "small things" big, you have a problem. Wiring a driver out-of phase and increasing the level (to that driver) could create an artificially huge image. Phil Specter's Wall of sound and many, many recording use the concept to successfully creat big, enveloping sound. Echo, reverb, phase are the tools engineers use daily to give your music scale! So the bottom line is: image size should be proportional to the recording. Your volume knob is your one real control--the louder, the larger things should sound. Magical recordings do it all: huge performance space, big scale, precise imaging, dynamic--the engineer gives you the majestic illusion of large sound. Thank him or her. Now go play with your knob.