Portion sizes have gotten larger in the last few years. A single bagel can actually contain two or even three servings. An individual bag of chips can easily feed more than one person. To control portions, read food packages to determine the number of servings before you begin eating. Serve yourself only one portion and put away the rest. In restaurants, ask for a “doggy-bag” right away and wrap up half your meal as soon as it is brought to the table.
Source: CDC. (PDF)