What makes speaker's sound big?

Mikelavigne, where you said....

some good mid bass is also helpful; again the speaker driver surface in the mid bass needs to be matched to the volume of the room so it can propagate. the whole room needs to be energized.

is there a formula  one could follow in order to size the speaker system to the room?

Hi, Martin (OP) 

Much wisdom spread throughout the thread here and elsewhere online. 

My 2 cents... 

For a great condensed reference that touches on all this and much more, you may want to get a copy of Jim Smith's "Get Better Sound" 
https://www.getbettersound.com/store 

Jim is one of the best in the audio / music biz and there are many things within its pages that may help solve your issue or at least improve the quality of what you hear. It is likely to help with other considerations and improvements as well. 

Although his in-person, in-home consultations are pricey, he offers very reasonable 30 minute phone consultations. The book and the chat might be the best investments you can make (or will ever make) in your system and setup. 

Cheers, 

Ken

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.