Size depends on BODY COMPOSITION, not weight.
You might think that two people with the same height and weight would wear the same size, but as you've all noticed, that ain't so. Sometimes it has to do with shape/fat distribution, and naturally different brands and even different items in the same brand aren't sized the same. Sizes are just NOT standard, they are marketing tools. (That's another topic for another day.)
But, say you had the same item of clothing and you were fitting two people of same height/weight/body shape. They still might wear different sizes. Why? BODY COMPOSITION, or fat/lean ratio. The more muscle you have relative to fat, the smaller you will be, because muscle is a denser tissue than fat. Also, things like bone density play into your weight / body composition.
So, someone like exercise guru Kathy Smith can be 5'9" and weigh 150 and be SMALLER than an unfit woman who is 5'9" and 150. As a bonus, the person with the leaner body composition needs more calories to sustain that extra muscle tissue, so they can eat more without gaining weight.
