What makes speaker's sound big?

If you want speakers that sound big try magnepan LRS speakers.
You've never heard anything like them. Neither have most others here.

Big sound.....
- Big speakers like the double impact (say no to bookshelf dinkies)
- Concentrics if set up right since they can produce deep immersive holographic soundstages
- dual subwoofers
- Lots of wood diffusion panels on front and back walls
- If you are not a 2 channel purist, set up a atmos/dts 5.2.2 system with front heights or 5.2.4 system (front and rear height speakers). Front heights should be atleast 2 to 3 ft above your fronts and 2 to 3 ft outside of front speakers. The big giant orchestra and humongous immersive 3D soundstages will show up in your lil room. Some very smart guys declared a while ago that 2 speakers (stereo) are not sufficient to replicate the acoustics of large venues and they were absolutely right. Science will give you big sound!!

In the car business there's a saying "there's no replacement for displacement" and while there are lots of technologically advanced turbo-4 engines these days that feel much closer to a bigger engine than their 2-liter displacements, a big engine still feels like a big engine. Similarly you can kind of predict how "big" a speaker sounds by the amount of driver surface area it has and thus the amount of air it is able to move. That being said, as others mentioned, for a given displacement some speakers will do a much better job of sounding big and full than others. I'm not sure I agree that it is about tweeter technology or dispersion pattern as much as it is about how the mid-bass response is tuned. That to me is the part of the spectrum that has the lion's share of the visceral musical material and can give a sense of ease and fullness to the presentation. So the choices the designer has made with regard to driver, crossover, enclosure, bass alignment etc. will factor in heavily. Often there is a tradeoff between damping and fullness that you have to balance carefully.

Some examples of smaller speakers that play big in my experience are the Sonus Faber Electa Amator, Silverline SR17 Supreme and Dynaudio Special Forty. They all have a slightly warmer mid-bass tuning and bass alignment that that isn't overdamped (in the case of the Dynaudio, I'd say quite a bit underdamped) that helps give a fullness and resonance that suggests a larger speaker. The Sonus Faber alone blew me away at RMAF 2018; add on a subwoofer (in that case a SF Gravis) and you have a huge, enveloping sound. I also recommend REL subwoofers as a good avenue to explore with a smaller speaker. 

Two subwoofers not one. Agree with mid-bass response point.
Yeah, no substitute for how it should be, either speakers or cars or whatever.

The final answer which on these threads is always, sub woofers. 
Sub woofers are like 400 watt amplifiers...why would you want more watts if the first watt sounds like crap. Why would you want to add subs to a system that sounds like crap..