The vest is adjustable to a point. The shoulder straps are adjustable and the cummerbund can "stretch". It's kind of tricky to explain how the cummerbund works so I'll just link the JPC's manual
http://cryeprecision.com/Resources/e...2-JPC(Web).pdf
Just remember that the sizing for the JPC goes by the size of SAPI plates, you can run swimmer plates on a JPC but there will be slight fitting issues