It is an indirect correlation. BMI is a factor related to the likelihood of having diabetes, and what's considered a safe range for white people is an unsafe range for Asians. Thus if you have a white person at a 25 BMI and an Asian person at 25 BMI, the Asian person has a higher likelihood of getting diabetes.

"The educated [Asian] population knows that they're getting diabetes and hypertension and all these things at a much lower BMI, but if you're in a culture where everybody's really fat and you're thin, you tend to go around and think, 'Well, I'm protected,'"