What makes speaker's sound big?

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.  

I have a pair of Ohm Walsh Sound Cylinders. They have an 8" driver crossed over at a high frequency to an upward-firing dome tweeter. They are capable of room-filling sound and 3-D imaging - from any listening spot! Bass quality is deep and full, so no need for sub's! Truly a remarkable design! 

Um, you have terrific speakers, and are looking for something worse. Is that right? And you expect to be able to get worse, yet somehow magically have it sound better. Alas, I only work in reality. Add a couple more subs, forget big you will be enveloped. I only go forward, never back. Sorry. But if you really are determined to screw up a good thing you don't need my help, plenty of others here happy to help you out.

I had the same experience as OP's with my KEF Reference 1s, they sounded great for jazz and baroque, but less satisfying for large orchestrations than the KEF Reference 107/2s they replaced.  So I carefully went through the setup of the Velodyne SMS-1 bass manager and the  pair of HGS-15 it manages, changing all the cables AND resetting the crossover from 40 to 80 Hz.  Magic happened.  The setup is as satisfying as ever for jazz and baroque, but now also has the spacious soundstage for large orchestrations.  A Mahler symphony sounds grand again.

db