As the story is told in John Love’s official history of McDonald’s, Wallerstein tried everything he could think of to goose up sales — two-for-one deals, matinee specials — but found he simply could not induce customers to buy more than one soda and one bag of popcorn. He thought he knew why: Going for seconds makes people feel piggish. Wallerstein discovered that people would spring for more popcorn and soda — a lot more — as long as it came in a single gigantic serving. Thus was born the two-quart bucket of popcorn, the sixty-four-ounce Big Gulp, and, in time, the Big Mac and the jumbo fries, though Ray Kroc himself took some convincing. In 1968, Wallerstein went to work for McDonald’s, but try as he might, he couldn’t convince Kroc, the company’s founder, of supersizing’s magic powers.
It's less shame than normative effects; if you have an x serve, that's normal. Two x serves is obviously more; but one x+1 serve is easily rationalised away. In modern pricing models the larger serving is certainly cheaper per amount of food, too, creating simple ways to create a situation where people are guided towards the correct choice (from a food retailer perspective).
Who knew creating compulsion was a part of food marketing? Oh right everyone who wasn't an idiot.
