Who looks at the largest size box of popcorn at cinemas and thinks, "I really want to eat that much popcorn in an hour and a half!"
Most are tricked into buying it (plus the giant fucking coke, more than you should drink in a week!) because it is perceived better value.
One of those big boxes is good for a group of people, and there is nothing stopping you taking the rest home.
Those supersize cokes are a bit small imo, given i can do about 2 litres of cider in the length of a film, I am sure I could do more coke.