well there are some physically big well known pianists out there 2day....ie. garrick ohlsson, lang lang....while i wouldn't call either of them particularly "buff," i don't c why being huge would hinder your piano skills in any way. lol
Lang Lang is indeed a "shrimpy Asian boy"...
Size does become a hindrance at some point. I'm 6'7", and my knees don't fit under any keyboard-I have to spread my legs to attack the pedals at an angle. It's bothersome, I envy those with shorter legs.I really should start exercising regularly. I do exercise occasionally, but daily exercises would certainly improve my (or anybody else's) general well being.