IN the fast-food industry, cleanliness may be next to godliness, but portion control is not far behind. To maintain financial health, a restaurant has to know precisely how much food it is giving to each customer. So McDonald's chicken nuggets, for example, are bits of meat, pressed and formed into shapes of standard weight, rather than actual pieces of chicken. Left alone, nature is just too variable for the bottom line.

Restaurants may understand the concept of portion control, but clearly, many of their customers do not. For as two reports from the Centers for Disease Control and Prevention pointed out last week, Americans are getting fatter and fatter. Say what you want about the impact of high-fat diets and lack of exercise, but the basic facts remain: Americans eat too much, and they're not doing much about it.

The reports, based on health surveys conducted in 1999 and 2000, showed that nearly two-thirds of adults 20 to 74 years old are classified as overweight (using a common height-to-weight index) -- as are about one in six young people ages 6 to 19. The surveys also found that about 3 in 10 adults are obese, and about 1 in 20 extremely so.

All these figures represent significant increases from earlier studies, the most recent covering 1988 to 1994. The prevalence of obesity among adults, for instance, increased by nearly eight percentage points from the earlier period.

But you don't need a statistician to tell which way the bathroom scales are tipping.

Consider the seat situation. A chair width of about 18 inches used to be enough to contain the average adult; now auditoriums, stadiums, even subway cars are installing new seats that are several inches wider. A movie theater chain that took out the armrests between pairs of seats so that lovebirds could snuggle now finds the seats filled with obese people going solo with a super-size popcorn and Pepsi. And Southwest Airlines recently reasserted its right to charge passengers for a second seat if they spill out of the one they purchased.

Americans like to talk the talk of svelteness, but they walk the walk of obesity. Diets and exercise equipment are obsessions, but they can be run through as quickly as a platter of buffalo wings on a football Sunday. Abdomenizer and Deal-a-Meal infomercials fill the airwaves, but a few months later, the Abdomenizers are hidden in the backs of closets and the Deal-a-Meal cards are the stuff of yard sales.

Low-fat foods have become a popular product category in supermarkets, but for many people, eating them does not result in weight loss. After all, there may be only a few grams of fat in a serving of low-fat cookies, but not if your idea of a serving is polishing off half a box while watching ''Survivor.''

It is also easy to convince yourself you're not eating too much when the label says it's low in fat -- so nutrition labels may encourage a sense of denial. And denial, the health statistics indicate, is rampant among the overweight. The rate of obesity from the doctor-conducted surveys of 1988 to 1994, for example, was more than 50 percent higher than the rate from a study in which subjects reported their own weight.

Still, in the same way that there are stages of grief, many overweight Americans have moved from denial to acceptance. Offensive linemen the size of heifers are no longer considered freaks. Overweight actors and actresses get starring roles on sit-coms. ''Plus-size'' models adorn mail-order catalogs. Support groups tell obese people to be happy in their skin.

And from quiet acceptance, a few people have gone to forceful action. The American Obesity Association, for example, criticized Southwest Airlines over its policy of requiring some obese passengers to buy an extra seat, and threatened to sue.

BUT perhaps no one has gone as far as Caesar Barber, a maintenance worker from Brooklyn who weighs 270 pounds and stands 5 feet 10 inches tall in his stocking feet (which, by the C.D.C.'s calculations, makes him obese). This summer Mr. Barber, who has had two heart attacks, filed a class-action lawsuit in New York State Supreme Court, claiming that McDonald's, Burger King, Wendy's and Kentucky Fried Chicken put his health at risk by not disclosing what was in their food or warning him about the dangers of overconsumption.

The food industry dismisses Mr. Barber's lawsuit as frivolous. But some legal experts liken his action to the early lawsuits filed by smokers against the tobacco industry. It took numerous cases and many years, they say, for courts and juries to finally accept that a cigarette company might be held responsible for what, until then, had been seen as an act of free will.

The concept of free will is something Mr. Barber has apparently only recently discovered.

Why not eat somewhere else, an ABC television interviewer asked him shortly after the lawsuit was filed. ''For the last couple of years I have,'' Mr. Barber replied. ''But before that, I used to eat McDonald's, Burger King. I ate the fat that they had.''

''They never explained to me what I was eating,'' he said.

Photo (Fred R. Conrad/The New York Times) Chart: ''Something to Chew On'' It might be hard for some readers to get their arms around this one: According to the latest data released by the Centers for Disease Control and Prevention, every third person on America's streets -- in its restaurants and bars, at its grocery stores and candy counters --is obese. There are endless theories about why this is so, from portion size and overuse of sugars and carbohydrates to flat-out sloth. Whatever the reason, the adage remains true: we are what we eat. Above, a meal ordered from a typical chain restaurant along with its caloric tally. At right: how the statistical cookie crumbled, then and now. PERCENT OBESE -- Prevalance of obesity in American adults Both sexes, age 20-74 1960: 13% 2000: 31% Men, age 20-74 1960: 11% 2000: 28% Women, age 20-74 1960: 16% 2000: 34%