“We are actively looking at serving size and evaluating what steps we need to take,” said Barbara O. Schneeman, director of the F.D.A. office that oversees nutrition labels. “Ultimately, the purpose of nutrition labeling is to help consumers make healthier choices, make improvements in their diet, and we want to make sure we achieve that goal.”

The push to re-evaluate serving size comes as the F.D.A. is considering ways to better convey nutrition facts to hurried consumers, in particular by posting key information on the front of packages. Officials say such labeling will be voluntary, but the agency may set rules to prevent companies from highlighting the good things about their products, like a lack of trans fats, while ignoring the bad, like a surfeit of unhealthy saturated fats.

On today’s food packages, many of the serving sizes puzzle even the experts.

For ice cream, the serving size is half a cup. For packaged muffins, it is often half a muffin. For cookies it is generally one ounce, equal to two Double Stuf Oreos. For most children’s breakfast cereals, a serving is three-quarters of a cup.

It is difficult to say exactly how much people eat, said Lisa R. Young, an adjunct professor of nutrition at New York University, but she said that research showed that the portions Americans serve themselves had been growing in recent years.

When it comes to cereal, she said, many children probably eat two cups or more.

Parents who glance at a box of Frosted Flakes and see that it contains 110 calories per serving may not realize that their children may be getting several times that amount each morning at breakfast.

Image The calorie and nutritional information on many foods may be revised if portion sizes are increased to reflect a heavier population. Credit... Paul Sakuma/Associated Press

“To consumers, the serving size appears to be inconsistent and unintuitive,” said Wendy Reinhardt Kapsak, senior director of health and wellness at the International Food Information Council Foundation. “They have trouble trusting it.”