Options. Different sizes, same price. I like it.
Two factors cause this: First you have the convince factor, and second is the placement of the items within stores. I buy smaller items all the time. I'm in the store and I want a drink and I see the smaller drink in the front of the store and I grab one. Not thinking about it I purchase it, drink it, and I'm done with it. I could always go and get a larger drink in the back for the same price, but I don't because the smaller size is convenient, and the larger size is located in the back of the store. That means I would have to go the back of the store to get it (lazy huh, but we all do it).
This could have a positive effect by being smaller. This means less sugar, fat, and calories, but it also means a negative impact on my wallet. Fast food would never have survived if we were all logical. Why would I go get a 12 oz blizzard when I could go get a half gallon of ice cream and candy for the same price. Luckily I can ignore the logical side of life and do what I want even if I know I can get a different size for the same price. It's nice to have someone make something else for me, or to have something in a convenient size.



