You can empty out cardboard boxes. Which is actually advantageous to do anyway--the cardboard boxes take up more space than the cookies/poptarts/cereal they contain, likely a reference to real-life manufacturers making massively oversized boxes for their contents in order to make it look like you're buying more.
Smart. It was these kind of details that got me playing.
Oversized packaging serves another purpose though: you only really see it with easily crushable foods: cookies or chips. Stuff like candy which isn't crushable almost always comes in more compact containers which don't have a lot of air in them. This difference makes it more likely that it's about cushioning products so that you're not buying a bag of dust. If it was a general "make it look bigger" thing then you'd see bags of candy puffed up like balloons too, but no company does this. I think people are more aware of the weight they are paying for than people say.
I can buy a big bag of choc chip cookies cheaper than ones that come with a plastic tray separating them, but the bag ones definitely have a whole strata of crushed cookies and dust in the bottom. With the tray ones you're guaranteed to be able to eat actual cookies and not be shoveling crumbs into your mouth. This is the reason people buy the tray ones, and not because the package looks bigger. So, if Cataclysm:DDA wants to model that really realistically, then fragile foods removed from their packaging shoud deteriorate into crumbs.