Average Japanese woman size?

I recently spent time with some foreign women in Japan and the conversation at one point turned to how intolerant or shocked Japanese are at their size. These women are not obese, but certainly they are larger than most Japanese women. So I went hunting for some statistics about Japanese women's size.

The [Japanese] ministry of health, labour and welfare's 2010 National health and nutrition survey found that 29% of women in the 20s were too thin (body mass index less than 18.5). (From The Times of India, May 25, 2012). America's ABC news reported the same statistics in this article.

According to Wikipedia, the average Japanese women's height is 158cm (Australia's I was surprised to read, is only 163cm). Which makes me close to, but still below average here.

I am not large, my BMI is about 22.5. Yet in Japan I usually have to buy shirts that are L sized.

I still don't understand why Japanese people, who tend not to say what they think straight up, have a tendency to quickly tell foreigners how fat or large they are. Ah, yet another mystery in this land. 

Meanwhile, I'm off to the gym, to keep myself from becoming a "fat foreigner". (No one has told me that yet, except when I was pregnant here and the nurse told me to stop eating fruit!)