Bigger drivers = bigger soundstage

Audio2design and Duke have it right. The OP is making assumptions that are incorrect. It all depends on the design of the speaker. Wide dispersion does equal more room interaction. The OP notices better imaging with one speaker but prefers listening to the other which is his prerogative. However, buying a speaker with larger drivers will not assure that he gets the same type of sound. 

The size of the baffle in which the driver is mounted is a big part of this too. There have been plenty of A'gon threads arguing about wide baffles vs. narrow baffles.

<<<<<<<<<<<<<<<<<<<<<<<>>>>>>>>>>>>>>>>>>>>>>>>

It is still all the rage. The problem with narrow usually means deap to get any volume for the bass drivers. That still leave WIDE reflective surfaces behind the front baffle.

Second; in lue of a place to mount a larger bass driver, they mount in the side of the enclosure.. The enclosure becomes unstable, I don't care what you do.. it will move side to side. What is left to do with this narrow baffle. A smaller longer throw, for the bass driver and mount it in the front or rear. The better solution is to remove the bass all together (my position), and move it to a different enclosure.. ALL of it..

The larger cone surface areas are not easy to protect from back waves. The reason for proper phase plug design.. I don't see it in ANY of current manufacturing. Maybe OPs older design had phase plugs, maybe not, BUT it would have made a lot of difference in distortion in the bass region, adding clarity in the upper regions..

The one manufacture that did do it, went out of business in 2012, VMPS. The guy made his own large drivers from the ground up.
Phase plugs were tuned for the driver application, the same way the magnets were.  Look weird, work great...

Cone strength.. WCF
Woven, carbon, fiber, WOVEN.. Not carbon fiber, Woven carbon fiber..
You can actually see through the CONE material..It has HOLES in it.. Shark skin.. goes against ALL you may think.. thousands of pin holes. 

Regards

Multi driver solution share heat between drivers and improves dynamic and decreases distortions. But it works for small derives as well for big drivers. 

If the biggest soundstage possible is the goal, The amp is the key.

So many rabbit holes herein. All worthy of explanation.  So much so hard to follow.

speaker designers will tell you it’s hit or miss when putting it all together.  
A rigid heavy speaker with ample cabinet room behind a driver with a mounting structure that don’t resonate, is key to transmitting sound.  Larger bass drivers help if they don’t distort, too small ain’t good.  A shoebox size speaker is clearly too small.