In terms of price, that's missing the point that an extra-large drip coffee or cup of tea might cost significantly less than small or medium of one of those complicated milk-and-coffee drinks. And even if someone is treating me, I might want a smaller drink than they're offering.

Cutesy names aside, I've taken to asking for a twelve, eight, or sixteen ounce cup of whatever, because even the simple English names aren't standardized. If I ask for a "small" or "regular" at Tea Luxe, they give me sixteen ounces; half a mile away, at Darwin's, "small" still means eight ounces, and sixteen ounces is a large.