that it's true that majority of japanese DO have so-called double eyelid. i lived there, and i felt the impression of their eyes were rounder and wider compared with other asians like koreans. don't know why, but i guess it's their mild temper shown in their faces.
though, i can't deny that there were really thin eyed japanese girls. i guess those beauty tools are just another great invention japan has made to satisfy all kinds of clients.