I mean, my guess is that it largely depends on the individual. How easily someone gains or loses weight is highly individual, and can be influenced by both environment and genetics.

To explain broader population trends, I provided some examples above of reasons why small asian women may have an easier (or harder) time staying thin. I would also add on in addition to these points that there may be a broader trend in asian culture to eat more reasonable portion sizes, or to identify junk food as treats only for special occasions - and these cultural standards may be passed on either explicitly (via instructions to children or pressure among peers) or implicitly (observing what others around you do).