I mean, first of all, not all asian women eat asian food. See: all the asian women living not in asia, who are 2nd+ generation immigrants. They are just as likely to be trashing their bodies at McDs as anyone else.

In general, a smaller body requires fewer calories to sustain than a larger body. And the human body typically accounts for this via hunger signals. Smaller people tend to stop feeling hungry before larger people.

However, it is a fairly well understood phenomenon that, yes, for people who want to meet the sort of "lithe" female beauty standard, it is often more difficult to maintain for shorter people since any additional weight they gain will be less distributed across their bodies than the same weight on a larger body.

Also, as you (kinda?) pointed out before (in a very almost racist way) - diet plays a huge role here, as a traditional asian diet will impact the body in ways that cause it to signal less hunger and burn off excess calories, while a modern western diet will do the opposite. But this isn't really a point for traditional asian diets, so much as it is a ding on modern diets. Basically any traditional diet that we can find around the world will help you maintain a thinner figure than a modern diet. There is no such thing as a traditional diet with twinkies and root beer.

And also, I lied before - asian women actually arent as likely to eat at McDs as anyone else. Due to historical and cultural factors, immigrants of asian descent in western nations are significantly more likely to inhabit higher socioeconomic classes than the average person in the same country. This status imparts on them cultural habits like not eating junk food (which is low class) and the economic ability to maintain these habits (eg, living near a Whole Foods and eating at a trendy restaurant, vs shopping at the gas station and grabbing some McDs). But probably more importantly, being in a higher socioeconomic class gives people a greater sense of security and general well-being, which reduces stress, which reduces hunger signaling and results in the body not feeling the need to store extra calories to protect against times of future hardship.

So the answer to your question is "well, it depends on the person, but broad trends indicate that asian women are not generally starving themselves to stay thin (any more than women of other races).