I suspect people develop a tolerance and their bodies grow accustomed to the large sizes. I see a guy at my local convenience store who fills up on a 24-ounce (700ml) soda and a king-size candy bar every morning. I swear he's gotten fatter over the last few years.

Of course, I'm usually getting a similar volume of coffee so I can't say anything...though in my defense, that usually takes me until 3pm to drink (I don't mind it going cold).