These sizings are based on the wearer's measurements, not the measurements of the garments.  

How to Take your Measurements

Bust – Take the widest part of your bust, which in most cases, runs along the tip of your nipple to your back, parallel to the floor 

Waist – Take the measurements at the smallest part of your waist. 

Hips – Take the measurements at the widest part of your hips, keeping the measurement tape parallel to the floor.