The girl on the right is wearing a size medium and stands at 177cm, the one on the left is wearing a size XXL and stands at 174cm. (info taken from the website these dresses are being sold on) I'm not trying to invalidate your concerns here and I'm sorry if my post made you feel disappointed, it's just that I'm pretty sure the one on the right is not a healthy BMI considering the sizes/measurements/look. And the girl on the right, while very slim, to me does not look underweight and going by the sizes/measurements I would doubt she is. You say you're close to underweight and still not that thin, but please remember that the way you see yourself may be vastly different from the way you see others.  Hope you're doing well!