I am skinny. At 6 ft and 135 lb (182 cm, 61 kg), my BMI is 18.3, at the edge of clinically underweight. I’ve been this size since I was 15 (I’m 23 now). However, in public opinion, I look scarily skinny because of my Pectus excavatum, or sunken chest. My sternum curls inwards, my lungs are tilted back, and my muscles hug so close to my rib cage that it seriously looks like someone took a shovel to my chest and scooped it right out!