As a consequence of carrying a greater amount of adipose (abdominal) fat, this, for some unexplained reason, causes insulin resistance. This means that the body's cells don't work quite in the correct manner to the insulin that is being produced by the beta cells of the pancreas. i.e. the body's cells can't take in glucose from the bloodstream as efficiently as a non-diabetic. (We normally get glucose from the breakdown of carbohydrates during the digestive process.) The rise in blood glucose levels is what indicates that a person has become diabetic. In the type of diabetes that you're talking about, type 2 diabetes mellitus, or non-insulin dependent diabetes, this is the link between obesity and diabetes. I will add, however, that not all obese people go on to develop diabetes, which indicates that there is something else going on in those that do. Genetics plays a very big part in this so, if you don't carry the genes for type 2 diabetes, you could still be obese, carry on eating what you want, slouch on the couch, etc. and still not develop diabetes.