Interesting. I would have thought soundstage was more related to the mid and higher frequencies, since our localization of low frequencies is very poor. If the 12" drivers were operating up into the midrange, their higher directivity at those frequencies could result in less room interaction, and a higher direct to reverberant ratio, which could be more effective at preserving directional information. Just a thought.
There is no substitute for cubic inch or in this case surface area
After listening to quite a few speakers, my conclusion is that if you want large enveloping soundstage, you need a lot of drivers.
I once had a speaker with two 12in. drivers and the soundstage is just floating in the air. None of my other speakers could do that.
Currently I have a pair of Thiel CS2.4. It is a very good speaker but with small drivers there is really limitation to what it can do in term of soundstage size. I really miss that.