... If you don't play loud, it won't be loud...
Exactly.
I have a concert grand in an ordinary room about 14 X 20, with a Persian rug, heavy drapes and several large oil-on-canvas paintings mounted so that they are average 2-inch depth from the wall. I find the sound simply enveloping--not overpowering (though that is possible, but even with ears that can hear the clock ticking two rooms away right now, I don't find the closing crescendo of Lecuona's "Malaguena" anything more than simply exciting in that room). Every nuance of sound is right there in my ears like listening to a good hi-fi with headphones.
In the next room, I have a high quality stereo system that cost more than most houses when I bought it 20 years ago, with all fiber optics digital component connections and four 1000-watt amps running a pair of 8-foot electrostatic speakers and a pair of subwoofers. I rarely use it. (I'm too busy playing to want to listen.) But yesterday, I was listening to a performance of "Rhapsody in Blue" that I've enjoyed over the years. What struck me was that my concept of what constitutes a good piano sound has changed. The system is as close to "live" as anything I've ever heard and I've (only once) had a neighbor from a block away come over to say, "I just love your music, but could you please turn it down!". (The more common response is "F*****g awesome" or "wow!".) Anyway, yesterday while I was listening to the Gershwin, I realized that while the orchestra sounded great, I no longer cared for the sound of the piano in that performance. It sounded positively thin and feeble amid that wonderfully textured and round orchestra in that large recording studio. All the subtlety and texture of the piano itself seemed lost in that setting.
My conclusion after listening to that familiar recording is that I have become so accustomed to listening to the fullness and richness of texture of even the smallest, quietest tones from my concert grand that a large grand in a huge hall no longer appeals at all. A large piano in a large room has to be voiced up so much to be heard over a large orchestra that the sound becomes positively brittle. But a large instrument voiced for a smaller room can have subtleties that you can't even imagine from playing a smaller piano in an ordinary room. Don't knock a big piano in an ordinary room until you've played a good one, well-regulated--and learned to play pppp levels well. A big piano has the power to amplify all the subtleties and a refined touch lets you explore an infinite tonal palette that you might otherwise never even know existed, either with a large piano in a large room or a small piano in an ordinary room.
I haven't been in many recording studios, but I'd bet a lot of solo recording work gets done on large pianos in rooms comparable in size to mine.