Recently I decided I want to get back into some pants I could wear a few years ago. They are a size 2. Inevitably when a small person decides she wants to be even smaller, she gets a lot of opinions. Mostly based on the very popular, but inaccurate, belief that size equals health. There's no question that being overweight contributes to a myriad of health problems, but there is no universal number for what is overweight. The question is what is overweight for a particular person. The same holds true for being underweight. There is no universal number. You can't make a determination based solely on the numbers.
Right now, I wear a size 4. And some people are insistent that not only do I not need to drop a size, but that I am too small as I am. But the truth is that I am a small-framed person. I'm not tall, my bones are small, and I'm not naturally curvy. If I eat a bunch of junk, don't exercise, and carry around 15 pounds of flab... I end up a size 4. In the process, I've abused my body with poor nutrition and laziness.
The actual size on my pants doesn't matter. What the number 2 represents for me is a time in my life when I was taking better care of myself. And if I eat well, gain some muscle, and lose some flab, then I'll fit a size 2, and I'll be healthier.
If you take good care of your body, that's the only thing that matters. Not the size of your pants or the number on your scale. You've got to find your own size 2. And stop worrying about mine.