What makes speaker's sound big?

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.  

@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.