What makes speaker's sound big?

You can achieve what you are looking for in a medium sized monitor and subwoofer preferably two.  

The differences within the speaker design (traditional box) that make for a big sound include-

1) Efficiency- higher efficiency = bigger sound / jump factor.  example Spendor A and D series.  
2) Coherence between drivers- perfect coherence and alignment of the drivers so that there are no gaps or suckouts.  The drivers sound as one.  example Spendor, Harbeth, KEF R series.
3) Dispersion.  Being able to disperse all frequencies in all directions has a big impact on sound stage size.  KEF, Harbeth, Spendor.  

Tube preamplification or amplification also helps.  

Subwoofers properly integrated is a must.  

@martin-andersen
What do you mean by "better integrated" bass? Again I harp on "requirements": Can you give us some more info pls? Do you not have enough? Too much? Perhaps you have a nasty bump in the frequency response someplace and it's boomy? Or bloated? Perhaps it's not a problem of equipment but room placement and tuning. I don't mean to sound rude. I believe that the issue needs to be clearly defined before a "solution" can be offered.
Happy listening.

Mark & Daniels. I have the 1st gen Maximus-Ruby. They use their own version of the AMT driver and a 5" woofer (crossover at 900Hz). Terribly inefficient but with a proper amp they have amazing dynamics and soundstage. The 2nd gen is supposedly even more refined. 

radiating area of drivers
dispersion characteristics of drivers
distance between drivers
baffle design
boundary reflections
placement

When talking about sounding big that usually translates to full extended bass. Powered subs are your friends there. Big = powerful and the bass is where most of the power in music occurs. It takes exponentially more power to deliver flat response as the frequency decreases and that power must be converted to long wavelength sound waves so that pretty much explains it. Tiny drivers/speakers alone are more challenged to deliver full extended bass and all fall short alone to some extent.