According to calcSD, the eastern average for erect dicks (measured bone-pressed) is 13.40cm (SD 1.45). 15 cm is just over 1 standard deviation from the average, so I guess it's not unreasonable for her to think that.
This doesn't hold true if you take the global average, which is 13.94cm (SD 1.67).
Which begs the question, how does she know what's big? Let's assume she knows dick sizes from porn. JAVs are censored, so she couldn't tell from that. Of she watched western porn, she wouldn't think 15 cm is big. So she isn't getting her standards from porn. The only logical explanation is that she has learned from real-life, in-person experience from Japanese men.
