When will the stereotypes end?
Smh
I think there are several routes by which some people have come to believe that "Asian women" (however one may define the bounds of that conceptual category) may have difficulty being penetrated by a "large penis" (again, however one may define the bounds of that).
The one that seems the most speciously plausible to me is the idea that Asian women must have "small pussies" because their average height is less than the average height of members of some other category of women. However, just as the correlation between male body height and penis size is very small (if any such correlation does in fact exist), any correlation between female body height and vagina size may be surprisingly small. (I will not even consider here the question of whether the size of a woman's vagina in an unaroused state has any bearing on her capability to "take a large penis.")
A second motivation for this idea that "Asian women have small pussies and can't take a large penis" appears to be many men's rationalization of their personal impressions of the demeanor and behavior of "Asian women" both inside and outside of the bedroom. I am completely at a loss to explain how one could logically connect (what some men perceive to be) a "submissive" and "demure" comportment with "having a small vagina" and "not being able to take a large penis," but perhaps this might hint that such men view even mundane social interactions through sex-colored lenses, and are fond of interpreting what they perceive to be "submissiveness" as being motivated by the woman's fear or trepidation of the possibility of being confronted by what she must somehow sense (correctly, of course

) to be the man's overwhelmingly awesome and destructive penis. The annoying (to me) whining and wailing of many actresses in pornography produced in Japan might be another tidbit that such men interpret as "evidence" to reinforce their worldview.
A third motivation (and the most ridiculously false IMHO) for this idea that "Asian women have small pussies and can't take a large penis" is the vulgar myth that "Asian men" (again, the bounds of this concept are not always clearly defined, but I assume that it refers to East and Southeast Asian men when used by a New World Anglophone and to South Asian men when used by a British Anglophone) "all have small penises." In my actual experience, East Asian men at least (granted, I have the most extensive experience with Japanese men in particular) have actually had larger penises on average than the men of most of the ethnic groups (Caucasoid ones) who are fond of propagating that myth. I suspect that most American/European/white/Caucasoid men who repeat that myth do so quite innocently or naively because of lack of actual sexual experience with other males (and probably because they have incorrectly presumed that body height is significantly correlated with penis size and therefore consider their casual observation of a lower average body height for "Asian men" as evidence that corroborates the myth), but there are probably some who repeat the myth maliciously, knowing full well that it is false or at least unsubstantiated.