Why is this funny?

In my opinion it isn't funny, except perhaps to people who think fat-shaming others is funny. But jokes are inexplicable in any case.
 
Humor is subjective. A better question would have been: "Why do some people think this is funny?" Even at that it might be hard for a person to explain if they themselves didn't see the humor.

Have you seen the long-running thread "Tell a Funny Joke"?
 
The only thing that really matters is whether the woman wearing it thinks it's funny. If she chose and bought it herself, and thinks it's funny, that's up to her. She might just see it as an amusing self-deprecating comment. I don't share her sense of humour but if we all found the same things funny, the world would be a rather boring place.
 
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