On the recent Esquire pants-size exposé:

Retailers’ facilitating the illusion that we are thinner than we are is a by-product of their chief goal, which is to force us to try on every item of clothing we are considering buying and let the endowment effect work its behavioral magic. Trying something on invests us in completing the purchase to a much greater degree—we’ve gone to all that trouble already and want something to show for our effort—and it also habituates us to the idea that we already own the thing we put on, and to not buy it feels as though we have lost something or had something taken away from us. So the sizes are just very vague guidelines to help us know which items to take to the fitting rooms.

Come to think of it, the endowment effect is probably another reason smart parents tell kids not to touch anything when they go in the store.
Vanity sizing for men < PopMatters