Compare the surface of a piano soundboard with the woofer surface of average speaker cabinets. From that perspective, it is only logical that an acoustic piano, even a small one, would fill the room more mightily.
Play PTQ via a PA system with ample bass, and you can get quite a bit of "rumble". Whereas it is not solely the low frequencies (which are actually very solid in PTQ) that make the "thunderous" impression on a real grand. I think there are cross-references (or -modulations) amongst various parameters and frequencies involved that may not have been perfectly implemented yet.
It has also to do with the exact behaviour of treble and midrange and a certain "saturation" or distortion effect (in lack of a better term) that a real piano can still sound considerably more aggressive, "thundrous" or "bity" than PTQ does (PTQ still being a bit on the "decent" side).
This is the same with every emulation I have heard so far. be it Hammond organ or brass -- most are already brilliant in many ways (like PTQ), but all of them seem to lack the last bit of "edge". Will they ever get this done? I hope so. After all, who knows where audio software programming will be 5ys from now?
This is, of course, not to say that PTQ today wouldn't already be a very expressive instrument. Just today, I had a jazz gig, and again it was a pleasure how exactly and dynamically this fine piece of software reacts to my playing. It would actually take an excellent grand piano to equal that (while that would, in today's situation, simply have been too loud to begin with).
Add some more "thunder" as requested here (or "dirt"), and we're approaching a piano larger than life.
BTW., playing with bass and drums, I am actually using a rather small speaker box (two 6.5" drivers + tweeter) and this is quite enough for moderate levels.