The glass of orange juice grew by more than 40 per cent compared to 20 years ago. That translates into 50 additional calories, or a weight gain of five pounds over the course of a year, if consumed on a daily basis. Dinner and lunch servings grew, too - 50 per cent more fruit salad wound up on the plates of the Rutgers students.

"People are eating with their eyes and not their stomachs," Schwartz said. "They're not listening to their bodies to tell them when to put the fork down."

Helen Guthrie did the original Penn State study when she couldn't get accurate self-reported data on how much food people ate because they wrongly estimated portions. That's when she set out to see if diners could visualize portion size.

Most people still lack that skill, but the portion size is getting larger, said Guthrie, a professor emeritus now living in Florida.

"The frame of reference for the serving size is increasing," she said. "They still don't have an ability to translate to amounts that are easily quantifiable if you ask people how much they've eaten. That has not changed."

Other studies have shown that people eat more when they are served more. Schwartz believes larger portions at restaurants and larger plate size and packaging all play a role.

Carol Byrd-Bredbenner, a professor of nutrition at Rutgers and a co-author of the study, said portion size is only one factor feeding the national problem of obesity.

However, people may not realize that they are eating more when they take larger plates and bowls.

"Plate size, bowl size, cup size are very deceptive," she said. "They can't estimate the amount of food in a dish and it makes it even more difficult when the dish is deeper or bigger."

A 1994 informal survey found that the standard plate size in the restaurant industry grew in the early 1990s, from 10 inches to 12.

"That holds 25 per cent more food," Schwartz said. "That really makes a difference in how much our plates can hold and how much we eat from them."

Obesity expert Barry Popkin at the University of North Carolina at Chapel Hill said the idea of "value pricing" in fast food restaurants, which sells much larger portions for a minor cost increase, has also changed perceptions at home.

"The most surprising result is the larger portion size increases for food consumed at home - a shift that indicates marked changes in eating behavior in general," he wrote in a study published in 2003.

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