Why? As MC said, body parts that have been trained tend to grow beyond their untrained counterparts, both in strength and size (since those two are usually connected anyway).
Do you doubt that a body-builder has bigger muscles than you and me? Do you think he was born with them that way?
I would need to make a video for that, since it's too easy to cheat in a photo.
My left hand is indeed significantly bigger than my right hand - a lot bigger, in fact - and (as you said) that's because classical guitar playing is simply much more demanding of the left hand. Sometimes the left hand physically hurts after playing hard pieces. I've always said that except for large wind/brass instruments, the classical guitar is probably the most
physically demanding of all common instruments.
And BTW it's not just the size, there is a muscle between the thumb and the index which is abnormally large at my left hand, yet only slightly bigger than normal at my right, the difference between them being huge. I've actually got a nice trick that I "perform" on girls at parties quite often - I ask them to hold the thumb of my left hand into one of theirs, and the index into the other, and to try to pull them apart. Almost none of them succeed!
I wouldn't try it with a man though, because a) men are, on average, stronger; b) no matter how abnormally large it is, there's no way a muscle between two fingers can compare with the power of a man's triceps combined; and c) they actually use more of their force, whereas girls are usually too impressed/scared to do so.
Anyway, I'm drifting off-topic. Sorry.
