It's a well known and documented fact that our culture discriminates against weight; the bigger people are, the harsher society judges them. And, thanks to the efforts of many, many people, this prejudice is more out in the open than it has ever been, and lots of us are working to change it. But...
what's not so often talked about, and should be, is that small/thin people can be subjected to judgement and negative stereotypes too. Here's an example.
I was talking to an acquaintance a month or so ago, a woman who is naturally very petite. Someone she didn't know had just approached her and said, "Wow, how do you stay so thin? You are so lucky. What size clothes do you wear? You must be happy all the time."
I had seen this interaction and observed the discomfort on my acquaintance's face as she struggled to think of what to say to the other woman. She ended up saying nothing, simply smiling, but she was glad to get to talk to me about the interaction.
She told me that people say things like that to her all the time, and it makes her terribly uncomfortable. She has even struggled at times to like her body and feel ownership of it, since people seem to feel so free comment and pass judgement upon her physical self.
She told me, "I didn't choose this body. I cant' help it if I'm little. I can't even find clothes that fit. And, I have a normal life- it isn't perfect, and I'm not happy all the time. I resent the fantasy people have about me."
We all glamorize being thin or small, but what this example shows is that our society can be ruthless to all of us regarding our bodies. In a sense, no one is immune- not men, not women, not big or small or muscular or average or whatever people.
Here's another example: about 10 years ago I was in a local grocery store when I spotted a client of mine in the fruit/vegetable aisle. She was far away from me and didn't happen to see me; but I saw several store employees talking and pointing to her. I could hear some of what they were saying. This client suffered from anorexia and was quite thin. The employees fluctuated between admiration for her and her body, jealousy and an almost hatred (their word, not mine) of her, and "disgust" (their word, not mine) because of her emaciated status. Talk about judgement! I felt so protective of my client, and so powerless to help her (I was, of course, because of confidentiality, prohibited from saying anything meaningful to the employees- I believed anything I said might have made things worse for my client).
It isn't until we, as Wendy says in her comment, change our thinking that we'll stop making these ridiculous "rules" about what is a "good or correct body" and realize there is no such thing.
(There is such a thing as having a physically healthy body. I'm not talking about that here, so don't misunderstand me. We all know perfectly well that weight can affect health- being too small or too large can affect our health. I'm specifically talking about the fallacy our culture has about having a "perfect body and being a good, successful person as a result.")