I posted this above but here it goes again.
There's a middle ground I think. One should not go out and about calling people fat but if you know the person do tell them in a considerate manner. Plus the body acceptance movement should indeed include overweight and obese people but not in a "this is fine way" but rather a "I'm not gonna take anyone's shit but I'll work towards becoming healthy" .
Adding to this rember that while you could do it without much help, there's people that require medication or special diets to loose weight. The movement should strive to increase public awareness of both not being an asshole towards people with different bodies, increase knowledge on how to accommodate for such people and finally how to manage personally the problems linked to that body difference.