How do you know? Is there a link you have or a study or a quote or something? Because while some people will just go with the megaultrahugemongo size drink, other people are all about not looking like the biggest glutton ever and so buy a smaller size. When they runs out they buy another one because thirsty man.
You should watch a programme called 'The Men Who Made Us Fat'. Big men in the industry were discussing it. Here is a quote from the BBC website:
'Speaking to Mike Donahue, former McDonalds Vice President, Peretti explores the history behind the idea of supersizing. 40 years ago, McDonalds hired David Wallerstein, a former cinema manager who had introduced the idea of selling larger popcorn servings in his Chicago cinema. Wallerstein realised that people would eat more but they didn't like the idea of appearing gluttonous by going back for seconds. By increasing the portion sizes and the cost, he could sell more food. In 1972, he introduced the idea to McDonalds and their first large fries went on sale'
