Study that question, my young ones, for it contains a miniscule shift of meaning worth the copywriter's weight in gold. Not "all this contains one hundred calories," oh no. Rather, our protagonist expresses the energy content of her snack in the language of spending. Once you adopt her viewpoint (or, in the argot of the ad biz, "internalize the worldview of the aspirational figure"), you come to understand that calories are issued--perhaps by God, perhaps by Richard Simmons--like a per diem stipend. When you've run through the 1800 in your wallet, you're done eating for the day; so you had better use them judiciously.
This conceptualization isn't completely off the wall--the same shift took place over the 20th century in our approach to time. We used to have time in abundence and found ways to "pass" it; now it is a scarce resource to be "spent." Turn the concept upside down and you have calories: we evolved to crave every last one we could hunt or gather, but now most Westerners can buy far more than is wise to consume. Our copywriter's brilliant twist was to reverse the polarity: the calorie is a measure of external energy to be consumed, yet the ad treats it as something internal to be let out, and that with reluctance.
The effect of all this is to transform calories--or, more to the point, avoided calories--into a sort of money. As such, they merit a status comparable with that of other currencies. And this transformation is fundamental to the mission of selling 100 Calorie Packs, which bear the central absurdity of costing more than the next-smaller size. In a world where cost is measured only in dollars or euros or yen, a rational consumer would never pay more money for less product. But where price is expressed as a function of two variables, one monetary and one caloric, the solution can be quite different.
That said, perhaps you are like me and want to give 100 Calorie Pack purchasers a good shaking, verify they can count to twelve, then present them with a box of Lowfat Ritz and some ziplocks. But that would presuppose the existence of a third variable: self control. And that, my young friends, is today in very short supply.