There are a number of reasons why it is wrong to put yourself down if you are overweight.
Judging yourself by your appearance is like judging a book by its cover. In effect you are judging your entire self by only one aspect of yourself. It's like taking your car to the wrecker because the paint is scratched.
Putting yourself down for being overweight is a form of bigotry. It is no different from putting yourself down for being black or Jewish. Bigotry is one of humankind's worst traits. You owe it to yourself and to the rest of humanity to refrain from this particularly vile habit.
Putting yourself down for being overweight is bad manners. You wouldn't tell your neighbour that she is 'fat' because you're not that rude, yet you call yourself by offensive labels.
When you put yourself down for being overweight, you are being unkind and unsupportive. Once again it shows a double standard. If your neighbour wanted to lose weight, you wouldn't put her down for it, you would show her kindness, encouragement and support. Why not do the same for yourself?
Putting yourself down for being overweight sabotages your weight loss efforts. By being overly critical of yourself, you diminish your confidence in being able to shed the weight.
By calling yourself "fat", you give yourself a label that you find hard to shake off. It's like giving a dog a bad name. Once you've labelled yourself this way, you see yourself as being stuck with the situation and unable to change it.
Losing weight won't make you a better person, just a thinner person. Similarly, being overweight doesn't make you a worse person, just a larger one.
Assuming that others will think less of you for being overweight is a form of mind-reading. While it's true that some people will judge you unfairly and unkindly, it doesn't mean that everyone will. The person that you think is putting you down, may in fact like your appearance. You can't read minds, so why pretend that you can?
Even if others do look down on you for being overweight, this tells us more about the bigotry of the judge than it tells us about you.
Our culture's obsession with weight loss belongs only to our culture. There are some cultures that prefer a fuller figure. To put yourself down for being large is to succumb to the fickle dictates of fashion.
No matter how thin you are, there will be some people who like you for it, some who dislike you for it, and some who couldn't care less. It is impossible to be liked by everyone. Even Mel Gibson and Julia Roberts have their detractors. You can't have everyone's approval, so stop worrying about what others think of you.
Instead of waiting until you lose weight to respect yourself, you are better off respecting yourself despite your weight, and then deciding to lose weight because you respect yourself.