Here's an interesting article, "Obese People Seem to Do Better With Heart Disease".
A good book on the relation of body weight to health is Big Fat Lies. http://www.amazon.com/Big-Fat-Lies-Weig ... 0936077425
You can get a used copy for about $2.50 plus shipping.
The book dispells many myths associated with being over weight. The book blast Covert Bailey's book, Fit or Fat. You can be over weight, fit and healthy.
In fact, Big Fat Lies research indicates that most individuals are healthier carrying a little extra body fat.
The book also profiles body types. Those with low body fat percentages are prone to certain types of diseases while those who carry a bit more body fat are prone to other diseases.
Your body type dictates which diseases you are most likely to develop. .
As an example, obesity may cause diabetes. http://www.wisegeek.com/does-obesity-cause-diabetes.htm
However, there seems to be some question if obesity cause diabetes or diabetes cause obesity, insulin resistance being a majory contributor to both. It's one of those, "Which came first the chicken or the egg" questions.