This is one which I ought to have noticed many years ago, and posted about it.
Is there a direct relationship between the level of technical proficiency of a pianist and the loudness of his playing?
I realised that few of my pianist friends who had excellent technique played quite loudly when I asked them to demonstrate a specific passage. It is not banging, but firmness in tone.
Take another example, I once watched Li Yundi teaching a masterclass with a younger girl. The piece was Chopin's Polonaise Op.26 in C-sharp minor. When the girl played, I thought the volume was acceptable, especially in the opening notes. When Yundi played, however, he played it with such gusto and volume that I literally jumped in my seat. His tone was certainly bolder, and of course, louder.
One of my theories is that since they are concert pianists, they are more used to playing at concert volume. As I was taught when playing the cello, what sounds like a forte to myself only sounds like a piano to the audience when played in a concert hall situation. I had to play louder to compensate for the difference in the perception of volume.
Of course, they generally have better technique, knowing how to exert force etc.
What I am really interested is the legends of Liszt playing the piano. Back then, the pianos were not as sturdy as the ones we have today, including the frame, the strings, keys etc. I read that Liszt would always break piano strings, and even break piano frames! Even if the pianos were not a strong as they are today, I think it is still an amazing feat to accomplish. It sounds like it would require a ridiculous amount of force to consistently break piano strings in many concerts.
I also read that Liszt would play so loudly that the seats under the audience would vibrate. I think it is still impressive today even a pianists manages the same effect with a modern massive grand pianos that Steinway produces. Yet, back then, the pianos were surely not as resonant as they are today. It seems simply amazing that Liszt would be able to produce such drastic effects with a piano.
On a side note, it would be unfortunate with be neighbours with Liszt!
Anyone care to shed some light? Maybe practicing pieces with full force (hopefully with proper technique) will assist in acquiring technique?

PS. Would be great if someone could share more legends about the piano playing of Liszt, like how he sight-read Grieg's Piano Concerto. I cannot find any documentaries of Liszt on youtube, unfortunately..