I think it's somewhat of a paradox: you can't be happy if you're suffering, and many overweight people suffer terribly, especially if they're so obese that it intrudes on every aspect of their life and degrades their health and well-being.

But at the same time, I think you can be happy even if you're "fat" in the eyes of others if you're not suffering and just a larger being than average. Because I know as a longtime overweight person that there's a toxic stereotype that you must be miserable (and somehow inferior) just because you're fatter than most.