The placebo effect is basically refers to the capacity of the body to respond positively to positive expectation. Usually the scenario is that one's symptoms will be alleviated by some medically related, but chemically inert prop-such as a pill-usually on the say so of authority such as that of a doctor.
Nocebo is exactly the same phenomena except when that expectation is negative. For some reason they are split into two as if they are different things.
The purpose of 'obesity diagnosis' is to be a nocebo effect. It is classified as disease, partly because science has no real answer to it but equally to ramp up anxiety about it, rather than because what is it merits that definition, which is the usual reason to define things.
It is meant to manipulate you mentally into feeling as sick as you can feel or even be, in order to fulfil the pre-destined diagnosis, such is the belief that it must be true.
How much this actually costs fat people's health is anyone's guess.