Remember, this is not a fat acceptance site, it's a size acceptance site. Everyone's body is different. Some people are naturally slim. That's just the way their body works. Some people have eating disorders and starve themselves, or binge and purge, and some of those people are slim, and some aren't.
Basically, if someone is slim, don't assume they have some kind of problem.
The next two links seem to have disappeared, but I have local copies cached here.
Oh, I found this link about a girl who is naturally slim and gets hassled about it.
Here's an article about Skinny Dissin', from chickclick.com.