Its annoying isn't it? Don't worry though, it will catch up to them. And if you are creating good habits now with exercise and eating right, you won't have a problem in the future when your body finally sorts its self out.
Also, weight by its self is not a good indicator of health. I have met some very unhealthy people who are skinny and weigh next to nothing. I have met some people who weigh a ton and are very healthy...
Ask your self, if you and your friends go out for the day and perform some sort of exercise all day long, who will burn out and quit first? You or them?