What makes speaker's sound big?

A small room!

OK lots of good answers and some not so good.  There are a number of key factors making speakers sound bigger:

1 - Design - many speakers that sound big have cabinets that are narrow allowing the music to flow around them.  Flat faced speakers not as much in my experience.

2 - tweeter position - top - middle etc., will sound different along with a rear firing tweeter.

3 - bass drivers add to a bottom and do aid in the perception but not always

4 - Equipment - how does you amplifier impact the sound.  In building amplifiers, I experienced an amp driving a pair of old AR speakers from the 70s or 80s come right out of the box.  More open sounding than many high end systems completely filling the room with sound.

5 - I have heard a customer who owns both the DI speakers and also the Spatial Audio speakers.  Guess which one sounds more open and room filling with depth to the soundstage?

Happy Listening.

You might like the Parker series with matching sub woofers. Awesome transparency with single driver! https://www.schmidtlambrixaudio.com/

In my experiences, speakers are like engines, and so many other things in life LOL. All other things being equal (room, cables etc.)  There's no substitute for cubic inches..

Try open baffle.  Incredible soundstage.  I have the Spatial Audio M3 Turbo S speakers with REL S3 SHO subwoofers.  The sound is incredible in my large room with acoustic treatments.

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.