This is a very interesting question. We need the physicists here to answer!
I've never actually measured the decibels on my piano. However, my understand is that there is a dynamic interaction between pitch and loudness, or should I say, "perceived loudness". Apparently, it works both ways - changing pitch affects perceived loudness and vice versa. At least to my ears, higher pitch gives a perceived louder sound than lower pitch.
Anyone else to chip in?