Well it sounds like you possess a naturally slight frame. If you were clinically obese, they'd have something to say about that too. If you do wanna beef up a little bit, I'd recommend eating larger quanities of healthy food such as chicken, fish, some steak, pasta, fruit, veg etc) and incorporating some weight training in to your fitness regime. You may find it takes a while, but the work will bear fruition. Good luck.