In addition to the super-size fries and big-gulp drinks served in countless restaurants, Americans are eating bigger and bigger portions at home as well, according to the first study to examine the issue.

Between 1977 and 1996, average portion sizes ballooned for salty snacks, soft drinks, French fries, hamburgers and other mainstays of the much-criticized American diet, according to the study, which is being published today.

The trend toward bigger helpings provides strong new evidence that a driving force behind the nation's obesity epidemic is that people are simply eating more.

"Portion sizes have increased over time, and it's not just outside the home," said Samara Joy Nielsen, who conducted the study with Barry M. Popkin of the University of North Carolina in Chapel Hill. "This is also occurring inside the home, which is not good news."

Although previous studies have documented a trend toward bigger portions in restaurants, packaged foods and even in recipes, the new study is the first to examine how much people actually eat, especially at their own dinner tables.

"Many people have said anecdotally, 'Oh, food portion size has increased over time,' " said Nielsen, whose findings are being published in the Journal of the American Medical Association. "Nobody's actually looked at whether portion size consumed had changed."

The nation is amid an epidemic of excess weight and obesity, which sharply increases the risk for a host of serious health problems, including heart disease, stroke, cancer and diabetes.

While lack of exercise is considered an important factor in the nation's weight problem, the new data underscore the idea that the larger portions people are eating play a major role, too, experts said. The growth in portion sizes has coincided with the rise in the number of people who are overweight or obese.

"You don't need any more explanation for the obesity epidemic. It's exactly what's causing it," said Marion Nestle, a professor and chair of the Department of Nutrition and Food Studies at New York University and author of "Food Politics: How the Food Industry Influences Nutrition and Health." Nestle doesn't reject the idea that a lack of exercise is playing a role in the obesity epidemic, but she thinks increased caloric intake is the most important factor.

The increase in the amounts people eat at home probably was spurred by the larger portions being served in restaurants, said Barbara Rolls, a professor of nutrition at Pennsylvania State University.

"Once people become accustomed to eating bigger proportions, they are going to learn that that's the appropriate amount of food. I'm not surprised it's carrying over to the home," said Rolls, who recently published a study that showed people tend to eat whatever amount is put in front of them.

Once a person has become accustomed to larger portions, it is hard to go back to more modest meals, said Larry Lindner, executive editor of the Tufts University Health and Nutrition Letter.

"The body has a really finely tuned mechanism for knowing when it's hungry and when it's full. But it can be thrown off. You can teach your body that it needs to eat more to feel full," he said.

For the study, Nielsen and Popkin examined data collected from two earlier surveys: the Nationwide Food Consumption Survey in 1977-78 and the Continuing Survey of Food Intakes by Individuals between 1991 and 1996. Together, the samples consist of 63,380 people ages 2 and older.

Between 1977 and 1996, portion sizes increased both inside and outside the homes for all categories except pizza -- an exception the researchers could not explain.

For example, the size of a typical serving of potato chips or other salty snack jumped more than a half ounce -- from 1 ounce to 1.6 ounces, which adds up to an additional 93 calories. Desserts increased from 4.2 ounces to 4.8 ounces -- an additional 55 calories. Soft drink sizes went from 13.1 ounces to 19.9 ounces, adding 49 calories. Hamburgers grew from 5.7 ounces to 7 ounces, for 97 more calories. Mexican food went from 6.3 ounces to 8.0 ounces, which piles up 133 more calories.

"Let's say you take in an extra 10 calories a day. Well, in a year, that's a pound. We're looking at increases of almost 100 calories for some of these foods. Chances are this will lead to weight gain," Nielsen said.