I think there are just always a few skeptical persons who deny even the most proven facts due to their personal or political reasons.
If somebody can´t accept the clear scientific proof on that US women have the largest average breast size, the reason of the denial is for sure one of the following 3:
1) She has not got too much in her own bra (=breast envy).
and/or
2) She wants to be “politically” overly correct on the cost of people´s freedom to know the facts. Even when everybody can see US women have larger breasts and research proofs the same, she can´t accept the fact that not every nation have the same breast size. It´s political talks, if anything.
and/or
3) she represents a nationalistic ´”ideology” which makes it too hard for her to openly accept the fact that women in her country have smaller breasts than in some other countries.
Or American women are just fatter on average. What about waist and hip sizes?
Seems like maybe this was just originally market research to guide clothing manufacturing. Other stuff about it is just social science navel gazing.