A majority of people who have diabetes are overweight or obese. This is not to say obesity causes the diabetes independent of other factors.
Many people are overweight and never develop diabetes.
Insulin resistance leads to diabetes in people who have a genetic tendency for this. Weight loss and increased activity are usually the first treatment strategies suggested for diabetic individuals who are overweight. With weight loss, fuel stores are reduced and cells become less resistant to the action of insulin. As a result, glucose clears the blood readily after meals, and fasting blood glucose levels fall. Diabetes is better controlled.