One of the biggest health problems in the United States today is the problem of obesity, the condition of being seriously overweighed. Half of all Americans today are overweight, and 22% are obese. And serving sizes are probably the leading cause. Serving sizes today have grown to enormous proportions. Fast-food restaurants serve “super-size” portions to show that they are giving people more for their money. And fast-food restaurants are not the only ones to blame. Almost any restaurant you walk into today will serve you a portion that is two to four times as large as the recommended serving size. In fact, today people consume about 234 more calories daily than they did 50 years ago. In the 1950s, a muffin was about 3 ounces: today, this popular breakfast food is about 6.5 ounces. Similarly, a portion of pasta today is usually about 3 cups, twice as much as restaurants in the 1950s served.
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According to the article, what is probably the major reason for the obesity problem?