All stereo systems produce a sonic image—some better than others. But, regardless of the image quality, there's one thing we can say they have in common: a size to that image.
If you turn the level of the stereo down to a whisper there's a tiny image trapped between the speakers. Crank it up to ear-splitting levels and it takes over the room. Somewhere in between whispering and splitting ears is the perfect level for every type of music.
One of the easiest ways to match the recording level with the proper playback level with the room is by ear. (I know, this drives some people crazy that we can't put a hard number on it, but it's an easy thing to learn).
Vocals and known acoustic instruments are the easiest to use for setting image size. We know what a voice sounds like and we also understand people don't generally have three foot wide mouths (though some politicians might). Adjusting the volume level to get the size of the singer's mouth exactly the right size garners you the perfect volume setting. If things change, like the number of people in the listening room, then further adjustments are needed.
I suppose we've covered this ground before but perhaps it's worth repeating. I routinely walk into rooms at audio shows featuring 100 foot long pianos or miniature people performing their music. I want to fix the sound by simply adjusting the level control, but I restrain myself instead.
Perhaps the exhibitor actually thinks a cello the size of a refrigerator is impressive.
Indeed, it is impressive but wrong.