What makes speaker's sound big?

Easiest answer ever:

MAGNEPAN

I realize that every manufacturer is not making tall speakers--wonder where they got THAT idea??

Try the best and the original.  Jim Winey was a pretty smart engineer, I guess.

Cheers!

Even though I know you guys like to shyte on MC, as an owner of a pair of Double Impacts I gotta say adding just one subwoofer so far has made a huge improvement. The sound got bigger and I think it brought out the mids and highs as well.
Been looking for a new house with a dedicated audio room. I plan on 2 subs when we get there.
From what I understand it will get even better.

My experience with the DIs are that they are big and they sound even bigger than they look. I dont think any small speakers are going to rival that aspect of sounding big. Different and possibly better. Sure .. but a sound coming close to rivaling them in sounding large, I doubt very much. 

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/