The economics of the "short" cappuccino

"Here's a little secret that Starbucks doesn't want you to know: They will serve you a better, stronger cappuccino if you want one, and they will charge you less for it. Ask for it in any Starbucks and the barista will comply without batting an eye. The puzzle is to work out why."

http://www.slate.com/id/2133754?nav=wp