The fast-food industry’s highly profitable practice of serving bigger portions has become a lightning rod for criticism by nutritionists and health advocacy groups who blame “portion distortion” for the bloating of America, a trend with unhealthy consequences.

Among other things, these critics are calling for legislation requiring restaurants to disclose calorie levels on menus, and are particularly critical of chains’ growing practice of “super-sizing” meals. They cite research showing that consumers who are served more tend to eat more.

The industry, however, says bigger portions should not be blamed for Americans’ growing girth, saying consumers are not being forced to overeat.

To help curb consumers’ appetite for tasty-but-gut-busting fast food, the Center for Science in the Public Interest in recent months has begun pressuring federal and state lawmakers to pass legislation that would require chains to publish the calorie contents of menu items. Such disclosures might give pause to many diners thinking of gorging themselves, said Jayne G. Hurley, senior nutritionist at the Washington, D.C.-based group.

“If you walk into a supermarket, you can pick up any package and find out how many calories and fat it has. But you can’t do that at a restaurant,” she said. “People have no idea what they’re getting, and would make better-informed decisions if they did.”

No fast-food restaurants currently put nutritional information on menu boards, but almost all of them, including McDonald’s, post the data on their Web sites. Burger King and Carl’s Jr., among others, display it in restaurants. Others distribute pamphlets at customers’ request.

The National Council of Chain Restaurants vigorously opposes any new regulations, said Terrie Dort, president of the Washington, D.C.-based trade association. “We don’t feel we need the government telling us what to do,” she said. “Consumers who want information can already get it.”

The science center’s Hurley said that was not good enough, arguing that consumers couldn’t go online while waiting in line for their burger and fries. Also, many consumers don’t even know they can obtain such information from restaurants.

McDonald’s, Burger King and other outlets push the bigger sizes because soft drinks and fries have high profit margins that boost the bottom line, said Carlsbad, Calif., restaurant consultant Hal Sieling. But they also add hundreds of calories to meals already low on nutrients and high on fat and sugar.

At $1.90, a super-sized fries at McDonald’s costs only 87 cents more than a small fries but adds 400 extra calories. But a typical super-size fries costs the restaurant only an estimated 15 cents more in ingredients than a small bag, Sieling said.

McDonald’s spokesman Walt Riker said restaurants were not to blame for the country’s collective bulge. McDonald’s offers consumers a wide array of options, including orange juice, salads and yogurt parfaits, he said. Steven Grover, vice president of regulatory affairs at the National Restaurant Assn. in Washington, D.C., said Americans were gaining weight not because restaurant meals have gotten bigger but because they were watching more television, playing more video games, doing less physical labor and have little understanding about nutrition.

“It’s too easy to blame the food,” he said. “People who have a weight problem are making bad decisions. Overeating is a choice.”

Perhaps. But studies have shown people will eat more than they normally would when served lots of food. Even diners disciplined enough to take home half their servings might be eating too much. Restaurant meals are often so calorie-laden and big that diners might want to consider eating only one-third of the meal to avoid overeating, said Larry Lindner, executive editor of the Tufts University Health & Nutrition Letter in Boston.