Why must we define our size with just one number?
I understand that it is not possible to make every item of clothing a company has for sale in every single possible combination of sizes, obviously it would be never ending. I also see that compared to men's clothing it makes sense, as they tend not to go in for all in one items*, and these numbers are 'dress' sizes. When trying to design and fit an item that covers 3 of the key measurement areas (bust, waist, hips) as well as the question of length in body, skirt and arms, it must be difficult to average it all out to a point where it fits the majority of women. Clearly the people in the know have had to perform some complex maths in order to create a univerally applicable sizing system that makes shopping a breeze.
But they haven't. Or they did, but it was decades ago and it's become largely obsolete with modern diets, lifestyles, style of clothes we're wearing and the underwear we are choosing.
*Perhaps the ubiquitous onesie may change this, should it prove to have sartorial staying power, but I doubt it. On a similar note I have never fathomed the logic of selling men's shirts sized only in relation to neck measurement, the smallest measurement you could base it on, except perhaps wrists. At least for women it tends to be your widest part that gets the deciding vote.