We are told from on high where to start our sizing, and work up from there. We start with sizes (small, 2, 23, whatever) based on research done on the customer for the particular store. Some stores have larger breasted women (how they figure these things out, I am not sure...) some have wider hips, some are shorter. Some markets will buy clothes very form fitting, some prefer a more roomy fit. It all varies not only from store to store but will often vary in the same store, throughout the US! (For example: major dept stores usually carry different fits and stock different sizes based on their market.)
We fit our 'size small' on a model who is 5'7, 25" waist. It would be nice to do a larger range of sizes but if research shows the majority of women are buying a 10, well, it does not make economic sense to create XXS-XS clothes for the increasingly disappearing customer market of thin women. Kind of awful, huh? It would be nice to go back to vintage sizing- we still learn to create patterns on a 'size 6' dressform- what we would today call a 0 or a 2.
If you are very small, a lot of Japanese clothing stores offer fairly decent shipping rates, and clothes that will be fashionable in the US next year. Weirdly enough, my favorite place for jeans and pants are Forever 21- although they changed them so they are not 100% cotton anymore, so that sucks.