You're skinny. LOL!

I'm the same height as you, and I was 150 lbs back in college when I was on the swim team. I had zero body fat. I looked at those old photos of me and holy crap I was skinny. My waist was around 29" and I had a hard time finding pants that fit in the mens section!

Having said that your body type has a lot to do with it as well. I have large bones that can support more muscle mass so at 150 lbs. that is simply too slim for me. I look much better at 180-190 lbs. It looks healthier and less like a skinny, 15-year old teenagers.