When it comes to fashion, most brands start their sizes according to the average size of their core customer base. If most of the people shopping at a store have an average waist size of 35 inches, that will be their default medium, and every other size will adjust to match that baseline medium size, a small will be 33 inches, an XS will be 31 inches, a L will be 37 inches, etc. You can't do anything about it if a company just sees a bunch of fatties buying all their L to XXL's and decides to market them as the new mediums, a lot of companies just want money and don't care about having a certain image or about catering to a smaller niche. I would suggest looking for brands outside the US if you're having this problem (like european brands, and possibly some canadian ones), and if you're extra small in size there's always the asian fashion market.