My BMI is ~20 and I could make my body look like that. Where is your BMI? Keep in mind that these girls are contorting themselves in ways that purposefully pull the skin taut against their bones and muscle. You couldn't see my spine/ribs like that just standing, but if I bent forward you could. These girls don't look like that just sitting around. Not to mention the fact that the artist probably played with light/shadows to emphasize their bone structure showing.