Something else to think about, is that the big movie theater chains, or at least their suppliers, have more buying power than the average public. As a result, the bigger, better, popcorn makes it way into the movie theaters, instead of into supermarkets. As a result, the popcorn you get at the movies is bigger and better than what you get at home.
Basically, it could be the same reason why you find that the bigger and better products go to restaurants, instead of grocery stores.
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