Why do I see this fat appearance when in reality I'm underweight?

A lot of people with eating disorders also have something called body dysmorphic disorder. Someone with body dysmorphic disorder does not see themselves as they really are. This is why you see yourself as much bigger then you really are.

Because your self image is distorted . You look thin , but your mind doesn't agree with that . Be positive and love yourself .

You might suffer from an eating disorder or have poor body image. I would seek professional help to get the care you deserve.

Because you're at a carnival in the house of mirrors or you're looking at an old portrait of yours instead of yourself. The saying "Perception is reality" only applies when one's perception isn't skewed by irrational beliefs If, as your question reveals, you know that reality is different than what you perceive, it's time to seek help.

This most likely has to do with body dysmorphia, which causes you to see a different, unrealistic version of yourself. It may have to do with past weight issues (weight loss/gain) or an eating disorder. There are other causes as well, such as criticism about appearance from others and harsh comments about how you look.

You have not given yourself enough credit. You have not truly loved yourself yet, and self-love is crucial.

Well, you have it in your head that you weigh too much which in turn changes the way your brain reads the images you’re looking at. You ‘see’ yourself as overweight so you SEE yourself as overweight! If you are worried about gaining weight, it’s best to do your research before any sort of diet, it’s relatively simple to put together foods which are nutritious which will keep you physically healthy and not make you become actually overweight. Personally, I’ve been taught quite a bit about nutrition! Weight is not as simple as counting calories! Stay strong!

Because you want to see yourself that way or you've got in your head that you look this way when you really don't.

Weight issues can be range from many factors from society and family. But the most important thing is not the weight its your health. There is no point/worth if you look so slim/thin but you are not able to enjoy it because you are in a hospital or always sick.