Vanity sizes are kind of annoying when you're trying to shop online though, I really hope they change that sooner rather than later.
I believe in the states, American Apparel is more catered to the "beautiful" crowd so their sizes are generally smaller compared to the norm ( Somewhat misleading I guess, smaller than American norm but alright by the rest of the world ). If I buy shirts in the States, I would have to buy small. In Canada, Hong Kong, basically anywhere else "and" places like AA, I would be wearing medium.
Makes it a tad confusing since a lot of indie T-shirt makers from the states that sell online use base clothing from AA and thus use their sizes.