Multiple views of the SmartVest demonstrate its "performance fit". Pictured is a Men's XS with 10 lbs of resistance. A young man with a 29-31" waist will wear this size. He might be 5'9" and weigh 145 pounds. He could be 6'2" and weigh 160. The side view says it all. Don't own a 30" waist? Not to worry, there's sizing for all body types. Guaranteed.