If your friends are your height and say they weigh the same amount but are that size, they're probably either lying about their weight, or HORRIBLY out of shape. Nothing is wrong with you other than needing to gain a few pounds, because you are definitely a little underweight. If they really are your weight and so many sizes bigger, you're probably just in much better shape, and have more muscle. That's certainly not a bad thing. Eat healthy, and probably more than you do now, exercise regularly, and don't worry about it.