Biggest Sound in the Littliest Box

The keyword here is the "Littliest" box. Have you heard the Tannoy mercury M1's? This tiny speaker is a bit taller than a regular magazine and smaller in width (300 x 170 x 210mm (11.8 x 6.7 x 8.3in)). Another tiny one is Dynaudio Audience 42 (11.1" x 6.7" x 9.7" (282 x170 x 246 mm)).

You might want to consider the Wilson Benesch Discovery. It's a fairly small monitor but the stand is integrated, as it is an important part of this speaker's design. So, I don't know, if that qualifies as 'little box' for you.
With regard to sound quality, I haven't heard too many monitors to compare them with, but regardless of size and price this is one of the finest speakers I've ever heard. No colorations, seamless integration of drivers, impressive bass due to isobaric loading (no need for a subwoofer) and wonderfully musical. They are a little bit out of my price range (8k, and they never show up used), otherwise I'd ditch my VirgoII for them.

"Big speaker sound?" Hummmm...that one might have to be quantified more for me. If you're refering to a bigger speaker as sounding more"full, "weighty", and "authoritive" in the bass reigion, then that's one thing. I've certainly heard speakers that offer a MUCH BIGGER SOUND(potentially, considering good room acoustics and set up) in terms of soundstage, than others!(refering to pair) Infact My older SF electa Amators and MInima Amators produced a HUGE SOUNDSTAGE in a proper set up! The sound was "bigger" than what you'd get from most any speaker, regardless of size! That Dynaudio Essotar 330 tweeter produced as much detail and life as most anything I've heard!
So, when you say "big" I'm just not sure what's being infered.
Also, I'd submit that tiny speakers, even if they have better than average bass response(if that's what's being refered to), will still need a powered sub somewhere to pull off the "full range thing" properly...otherwise you've got a limited aplication monitor...if that's what you're after.(which for some applications is ok) That said, if you're after a monitor that produces a big sound(probably refering to size of soundstage) within its respective frequency response, and you add a well set up woofer somewhere,you should have some overall "big sound"!
Of the "tiny" monitors out there, I have a hard time finding "a pair" that produce a HUGE SOUNDSTAGE personally. You might consider doing multiple minimonitors in a multi channel set up for that.

I would bet that if someone could build a smalle speaker cabinet to house a dynaudio Essotar 330 tweeter and high end 4" driver, and integrate a proper crossover(challenging?), you'd have HUGE SOUNDING MONITORS when mated with some well set up good bass woofer(s)!!!

Quad 12L...and real bass to boot...I cant get that excited about any monitor that barely does 60 hz...go full range...or dont go at all...