Why everyone is biased against obese people?

Why an obese person can't show off their body in the beach without others stare once a fit person does it proudly, or an obese person can't wear a tight shirt while a fit person does it with no pulic stigma, or they can't eat two meals for lunch at work if they feel hungry still or if they like to without having others stares. It's gone far to the point that even people pray that they don't have to sit next to an obese person in a plane, which they have paid as well as them.
I don't know when the society loses these biases and accepts the differences, Do you really know those stares are really sharp and rude? Because if I'm proud of my body of my weight others have to respect it, too.
Why everyone is biased against obese people?
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