In theory, it isn't supposed to be. Petite clothes are supposed to be shorter, but also smaller in general. Narrower shoulders, smaller openings etc. For example, I buy petite pants from a particular brand and they are several inches shorter, but they are also 2 inches smaller in the hips and waist, like the 0P has a 23 inch waist vs regular 0 25 inch waist. However a lot of stores now just make petite about height which is why you can find "petite" size 18's.