penny-wise and pound-foolish
Where someone compulsively saves money by buying super-cheap-priced foods like crackers and white bread, not caring/realizing that these salty/sugary/"empty-carbs" foods will make him gain unhealthy weight in a hurry. Ben Franklin was always sagely expounding about stuff like this, and just look at da deplorable "tubby" shape HE was in!
That balloon-bodied dude with clogged arteries in the ER never eats health-food stuff "'cuz it costs too much" --- talk about "penny-wise and pound-foolish" --- what good is being a penny-pincher and "fattening" your bank account if you're also gradually fattening YOURSELF in the process???