Perhaps the logic is that most people will only go for the supersized option purely because it's on offer, and that's a large factor in the increased obesity in the population? Just speculating, of course, but I know that I'm always tempted to get a larger size coffee, even when the regular would be just fine. Also, the price of two half-size drinks would be more expensive than one super-sized, so perhaps price would put people off too.
There certainly are better ways to tackle obesity than this, but I can't say I'd be too hurt if such a law were to be imposed over here if it proved successful.
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