My girlfriend is 5' 10" and 130 lbs.. the medium pack suits her great. I'm a skinny (and tall) male and the medium works great for me as well. Just based on anecdotal online info, skinnier hips and people seem to like this pack.
For eg. larger people than me seem to complain (online) that the bottom of metal frame pushes against their back while it doesn't really touch me much.