The problem is they conflate "healthy" with "absence of blood pressure/cholesterol/beetus problems" and assume that if the latter is the case, the former is also the case.  Just because you're a hamplanet with normal blood pressure and normal-range cholesterol levels, doesn't mean you're healthy.  Your extra weight is ruining your body, it's wrecking your joints, your fat deposits could be damaging your internal organs, and so many other things. It is absolutely impossible to be fat and healthy.