It's not a moral question at all. It may be an aesthetic one, or a question of which body feels better to live in, but it has nothing to do with morality at all.
P.S. I think you might have to take a step back and explain how you are defining morality. Bodies and food would not seem to fall into the realm of good and evil as I would define them. Becoming so overweight that you could not function or so thin you had to be hospitalized (at the extreme end of this spectrum) would still not seem to me to be morally problematic but rather a kind of illness. Is a person with cancer immoral or just ill and in need of care? In the middle ground, an overweight or underweight (however you define these) is no more "immoral" than a child with messy hair. In need of better care, maybe, but not immoral.Karen was rejecting moralizing about food and bodies from a reductionist stance -- that there is a lot of complexity around food and bodies. Even though I may seem to have missed the point, what I'm saying is that it really isn't a moral question at all. Maybe I am the person I most need to convince myself of that truth, because I think on some level I do judge myself (though not other people) harshly in terms of my body.
I'm not saying it's all the same. Personally, I know that my body feels its best within a certain range, and functions best. Right now I'm dealing with some things, like GERD and arthritis in my toes, that I think would be ameliorated by losing some weight. That isn't a moral judgement and I don't think it means I'm not adequately self-loving. I think that gently moving toward a lifestyle that will support a smaller body is going to be an important part of self-care. It's not about confessing my sins at the scale and doing the penance of 100 bowls of plain steamed broccoli and 90 situps. I gave up on Catholicism as a religion long ago and there's no need to bring it into my diet.
If I want to do a moral act, I'll give some money to charity or bite my tongue instead of saying something hurtful.