How to make small room sound bigger

You cannot make the room sound bigger.  If you do a comprehensive acoustic treatment along the likes of what erik-squires suggests, you will get smoother frequency response, better dynamics and better soundstage/imaging.  A larger sound image in the same size room is the best you can achieve.  It will involve both absorption and diffusion along with a good deal of trail and error experimentation.

@ei001h 

I have that exact room size but with carpet over concrete. I did what @erik_squires  suggested, called GIK and worked with them to treat the room properly. I ended up with 12 of their 242 panels (4 on the front and back walls and a pair per side wall) which made a significant positive change to the sound of my system. Now, I can and may go further with GIKs suggestions but at the time it worked with my budget.

My system: Anthem STR Stack> Aurum Cantus V7F & Pair of SVS 3000 micro subs

Good topic  OP. I keep toying with the idea of turning my office into the listening room. It's 14 x 13 w 8 ft ceilings so very similar. Thanks @erik_squires  for the GIk recommendation. That looks like a good source for a DIYer

Yes, by all means consult with either GIK or ASC or both (or another acoustics company). If you are charged anything for their work, it will be refunded if you order, and it will be worth the price if you don't. There is some knowledge to be gained by seeing what more than one company recommends.

Everyone has run into successes with room treatment and a few things done wrong. My error in the latter, many years ago, was adding too much pure absorption to my audio room. That created an artificially dead feeling. The error can be avoided by using combination absorber-diffuser products (as already recommended) predominantly. Examples are GIK's Alpha series, the RPG BAD products, and most products from ASC.

PS - You can use the room mode calculator here to see where your problems are going to be:

https://amcoustics.com/tools/amroc?l=14&w=11&h=8&ft=true&r60=0.6

The first 3 modes are excellent candidates for bass traps in the corners. The 70 Hz mode is now floor to ceiling and using narrow mondo traps horizontall at the floor and ceiling behind the speakers and listener help here. At 90 Hz and above you’ll want to have wall panels spread out but these are a lot easier to deal with (i.e. smaller traps) than the first modes.

Further, try to keep your subs out of the pressure zones of the modes below 80Hz.  Not possible for every mode, but if you can avoid 3 out of 4, it's a win.