In addition to what
Aphrodite has said, there's another issue here: Asian women are not slim and smooth-skinned and short and cuddly by default. Most of them get this way because they have a good diet, and usually because they exercise all the time. While you might not notice it, there's a quite sizable number of fat, pimply, and tall Asian women in this world. This is especially true in the
USA (and probably
Canada as well), where a typical suburban lifestyle can easily make you fat and pimply and tall unless you make a conscious effort to keep yourself in shape. It's also starting to become a problem in
Japan, where girls are eating
processed food and playing
Playstation and skipping
PE.
Still, our stereotype is of the fresh off the boat Suzy Wong type who's been living on a diet of white rice and soy all her life. So many white guys see skinny, short, smooth-skinned Asian women, and get the impression that they're drawn to the race. Usually, they aren't, because an Asian woman without the body will just turn them off, and a comparably embodied woman of another race will get the same penis-jerk reaction from them.
While we're at it, if you think you're attracted to Asian women, find a decrepit 70-year-old in the back of a Chinese restaurant and see if you can prove yourself right.