Bien dans sa peau
"Feeling good in one's own skin" is an very common French expression.

For me,to feel good in my own skin, I need to be lean. When I'm carrying extra weight, my body feels foreign, I don't feel like "me" and thus, my main point here, I feel bad in my own skin. And feeling bad in one's own skin means, at least for me, feeling uncomfortable altogether in myself. 

There's lots more to say about this. For instance, if feeling good about myself is dependent on my weight, how shallow is that? How much am I allowing myself to fall victim to the you've-got-to-be-slim-to-win longstanding prejudice in our culture? (Our "overfed" culture, I might add, which is really crazy-making.) On the other hand, if I've gained weight because I'm overeating for emotional reasons, and because my body has a propensity for craving high-calorie, low-nutrient sugary, white-flour foods -- perhaps leanness, for me, is a necessary state for true physical, mental, and spiritual well-being -- youth and size-0 culture be damned.