Thin people with T2D are probably “TOFI” — thin on the outside, fat on the inside. There is a concept of the “personal fat threshold” which is how much fat you can pack into your fat cells, and how many new ones you can grow. This concept explains why Asia and India have high rates of T2D at a much lower BMI.

An interesting way to show that this is likely the case is people with lipodystophy - they have no subcutaneous fat, and they are almost all diabetic.