I think the Big Gulp and the rise of the supersized soft drink in general is all about the perception of value. The illusion of value, more accurately. A big cup is already pretty cheap, but hey, a gargantuan one for just a little more, relatively speaking, seems like a huge bargain, a bulk discount. In reality, the cost of pop to the retailer is already incredibly low - way, way less than the price charged to the customer - a profit margin in the hundreds of percent. Fast-food outlets, movie theatre concessions and convenience store chains make more profit on cups of pop than almost anything else.
Big Gulps and Rob Ford are all about the appearance of being smart/careful with money while making an unnecessarily large purchase whose real cost is hidden.