What makes speaker's sound big?

placement, toe in and seating position can tweak soundstage.

A great sounding room.

There a a million factors that dictate how big a speaker will sound.  The key factor is output coming out of the speaker, not in terms of SPL or sensitivity but dampening.  A box that is well damped will have minimal cancelation inside.  

Tweeter dispersion is incredibly important.  Ribbons and AMTs that spread around sound will sound bigger than a single dome tweeter.  

Do they rely on room reflections to build the soundstage or do they sound better when the room is heavily damped?  How is your room designed.  A speaker like MBL Radiastrahler 126s will sound massive in a room that is energetic but will disappear and sound small in a room that is over damped.  

The crossover design impacts this.  I find that the steeper the curve the less energetic a speaker is and the more you need big drivers, arrays and/or tons of power.  

There are a million different ways to accomplish this.  Open baffles, electrostats, composite cabinets, integration of room treatments, etc...

It really depends on what you want, aesthetics you can live with or find exciting/interesting and what your budget is. 

An extreme example is the Wilson-Benesch Endeavour.  It is a stand-mount speaker (that is $50K) but is an incredibly well made cabinet and has a monstrous output in terms of imaging and soundstage. 

Another example is Audiovectors R series with rear firing tweeters and/or midrange drivers depending on the model.  These sound much bigger than they are.  

In the end, they may not be complete substitutes for a large floorstander but you can get close enough to be happy.