For food companies, the actual monetary costs of offering larger portions are small, because the cost of the food itself is small relative to labor, packaging, overhead, transportation, marketing, and other costs.
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At movie theaters, upgrading from a small to a medium-sized bag of popcorn without butter costs just 71 more cents. However, it also costs an additional 500 calories (i.e., a 23% increase in price buys 125% more calories). If you shell out another 60 cents, you can get a large, which brings the total to 1,160 calories and almost three days’ worth of saturated fat … At McDonald’s, the difference between a Quarter Pounder with Cheese and a Quarter Pounder w/Cheese medium Extra Value Meal is $1.41, 660 calories, and 4 grams of saturated fat.
Earlier here.