It did not look like the same fit on me as on the super skinny model, (I am 5'5" 160lbs) but im crafty, and removed the fabric that connects the straps in the back and used it to add some length to the shoulders.  Making the straps longer gave it a great look, moving the bulky area down past my hips which is the look I was going for (like the model)
So, depending on body type.. one size would be fine