General Discussion

They aren't. It is strongly hereditary - much more strongly caused by heredity than by weight, and there are probably multiple varieties of at least T2 diabetes, but doctors treat them all identically.

Both my family and my spouse's have diabetes.

My maternal grandfather - and everyone who is descended from him who is my age or older - has diabetes. It is a pretty benign version of diabetes (not linked to heart disease, regardless of cholesterol levels (not a single heart attack or other heart disease; my current cardiac calcification score is 0% calcification - and I have high cholesterol); not linked to weight (the BMI range in the individuals with diabetes ranges from underweight to obese). I am not aware of any A1C in the bunch that was ever higher than 8.0 on less-than-maximum metformin as the sole medical treatment. Pre-COVID, all I had to do to have a normal A1C was watch what I eat. That applied at any weight- including when I was obese. (COVID messed up my ability to control it entirely by eating, but my last A1C was 7.1 - high for me, but still within an acceptable range for my age.)

My spouse's maternal grandmother had diabetes. While it is not as universal in her descendants, many have it. It is anything but benign. My spouse's A1C is currently 10.1. She is skinny as a rail, and on three diabetes medications (Januvia, glimiperide, and metformin), and her A1C has not been less than 9 for over 2 years. Her sister is faring better on control - but also skinny as a rail. Grandmother was in the normal weight range.

I don't know the entire family history of a friend of mine, but she has been obese nearly all of her life with zero signs of diabetes - until cancer destroyed her pancreas. Post pan-can, she is T3 diabetes.

It may well be that some diabetes is linked to weight - there is certainly a statistical correlation, but (1) a statistical correlation doesn't mean that everyone who is overweight will become diabetic and (2) even you fall in the group contributing to the statistics, it is likely some underlying genetic makeup gave you a predisposition to both, rather than that the weight caused the diabetes (and there will be people who are overweight - like my friend - who don't have the underlying genetic makeup and will never get diabetes absent something like pancreatic cancer, and people like my spouse who become diabetic despite being underweight).