Why is being overweight wrong, but being underweight is totally fine?

RainbowFanGirl
I saw a question on here a few months ago,
and the girl said she was 5'7' and 96 pounds or something like that. She asked if that was okay, and guys said that was their preference and that she was totally fine. When people said that she was underweight and needed to gain some weight, they were downvoted and totally ignored.

I was just wondering... Is being underweight bad too? I see overweight people post pictures too and people don't hesitate to tell them that's wrong. Just curious.
Why is being overweight wrong, but being underweight is totally fine?
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