Do you have a source for that, or do you just expect them to want small phones because of small hands?
The rest of Asia are the front runners of large smart phones, so I don't see the small hands thing being relevant to phone sizes.
To my recollection, phones have always been pretty big in Japan. When I first visited in 2006 I asked my Japanese friends why all the phones where so big, and they just said "It's fashionable, nobody wants a small phone", but I've never noticed a trend towards small devices, just more features and bling.
Moreover, given that Japan sees relatively little usage of computers, but historically has been extremely cellphone oriented, it actually makes sense that you want a large screen as your primary screen.
Lastly, I've noticed that young people on variety shows, when doing a gesture for "typing", swipes the air with their index finger over the palm of the imposing hand, suggesting that the primary mode of text-input is on (1) your phone, and (2) this is done with 2 hands. I don't think Japan has a special affection for one handed usage, at least not since they moved to touch screens.
