He's just big: she's fat.

On the same diet, men drop nearly twice as many pounds per week as women do, according to endocrinologist Donald Smith of the Mount Sinai Medical Center in New York City.

Not because of superior willpower, as suggested by men questioned in a recent exit poll. (They were exiting my kitchen.)

Because men and women are built differently, they burn calories differently. Basal metabolic rate--the number of calories you need to stay the same weight and fuel functions like digestion and heartbeat--gets higher the more muscle you have, Smith says, because maintaining muscle requires more energy than maintaining fat. That means the average-size man, who has more testosterone and therefore more of the muscle it builds, burns fully one third more calories just sitting there (1,900 a day) than does the average-size woman (1,430 a day).

Male or female, a body must burn 3,500 stored calories of fat for each pound of weight loss. Put a man and a woman on the same 1,000-calories-aday diet, and the man, he of the more voracious metabolism, will start dipping into his fat stores sooner.

Another reason most men lose unwanted weight so easily once they decide to: They' ve rarely dieted before. "We have a great deal of data showing that the more frequently you've dieted, the harder it is to lose weight later," says Yale psychologist Judith Rodin, author of Body Traps. "Most American girls start to diet in junior high school; men diet later in life and less often."

Rodin's colleague at Yale, psychologist Kelly Brownell, has found that yo-yo dieting--the cycle of losing, then finding, the same 10 pounds every few months--seems to rev up an enzyme called lipoprotein lipase, which encourages fat storage. "After setting people on aggressive diets-- some that were as low as 800 calories a day--who weren't losing weight as fast as they should have," says Brownell, "I got the idea that the body starts to defend itself against the threat that dieting represents, which makes successive dieting more difficult."

Since yo-yo dieters are usually women, who learn early to feel that their bodies don't measure up--or rather, measure down--it would not be a reductio ad absurdum to say that men lose weight more easily than women in large part because of an insidious cultural double standard.

"Boys tend to gain their puberty growth spurt in muscle, and girls gain theirs in fat," says Rodin. 'That' s biologically determined. Boys' pubertal development brings them nearer to the societal ideal for men, but girls' pubertal development pushes them further from the societal ideal for women. That's culturally determined."

A national survey in the late 1980s showed that 90 percent of American boys and girls were unhappy with their weight--but boys wanted to weigh more and gifts less. By the time they're 13, a majority of American girls--but not boys--have already begun to diet.

The self-image discrepancy persists into adulthood. A number of studies show that women often judge themselves to be overweight even when they're not, according to the most liberal height and weight standards, while men come to believe their weight is just fine even when they're far larger than the norm (or far larger than Norm, on Cheers).

"Not only do they [men] think they're cute," says Rodin, "but society tends to treat them as cute no matter how they look. Men aren't usually punished for being overweight. The adjectives describing large men--imposing, for example--are often more favorable than those describing large women."

"The good news," says George L. Blackburn, a Harvard obesity specialist, "is that for the same amount of health, women can have twice as much fat on their bodies as men can.

The healthy allowable percentage of body fat in men is 10 to 15 percent, and in women, 20 to 30 percent." Well, OK. But the bad news is that so many women berate themselves for that healthy fat.

Here is a montage of the American woman' s changing form over the past century and a half: In the mid-nineteenth century, corsets created the hourglass figure, emphasizing hips and breasts and torturing internal organs; the addition of the bustle parodied the shape of ancient fertility goddesses. The buxom Gibson girl, however, gave way to the Jazz Age flapper, with bobbed hair and Bobbed body: slim, hipless, breasts bound flat as a boy's. During the thirties and forties, meatier was the message; shoulder pads helped create a figure signaling that Rosie the Riveter was up to any task. But when the boys came home, the ideal girl came round again--the Marilyn Monroe Modified Hourglass. Exit Marilyn, enter Twiggy. (Some women alive at the time still experience terrifying flashbacks.) And now? The High-toned Woman, she of the Linda Hamilton Terminator Arms.

And note that through all this men's bodies didn't change much. Portly lost its cachet by mid-twentieth century, but basically there remained the same large range of acceptable physiques.

"A culture's rules, gender difference, hierarchy, and beliefs are written on women's bodies," says Ann Bolin, an anthropologist at Elon College in Noah Carolina, who studies women bodybuilders. What are we, subway cars? Why do men' s bodies stay cultural-graffiti-free? Because traditionally men have done the cultural spray painting. Men have been judged by what they do, women by how they look--and what their looks say about Man the Doer. A plump wife advertised her husband's wealth and power; her hourglass figure clearly defined her job--bearing children. The thin thing is more complex.

"One interesting theory," says Bolin, "is that during periods of liberation, like the 1920s, when women had just gotten the vote, and the 1960s, when the Pill became available, the ideal shape for women deemphasized their reproductive characteristics--the nourishing breasts, the wide, childbearing hips."

But the skinny ideal has also been interpreted as a form of subjugation. The Pill gave women more control over their sex lives, and changes in the workplace have given them more control over their economic lives. With power over reproduction and production slipping away, the reasoning goes, men are keeping the upper hand by controlling consumption--insisting that women aren't thin enough.

"But is this really an issue of men dominating women?" says Mark Nichter, a University of Arizona anthropologist involved in the Teen Lifestyle Project, a long-term study of teenage girls, including their attitudes toward weight. "Or are women, by adopting the desire for thinness, actually taking control of their own bodies?" If it is male domination, Nichter asks, "which men are we talking about: what race, class, income? Is it men at large, or the fashion and advertising industries? Where's the data that shows this is what men actually want?"

Anne Bolin's study of body image has left her with questions too. "Is dieting a denial or an embrace of our passions? Does getting in shape defy the patriarchy or play into its hands? When women get out and exercise, they may be striving for slender bodies--a form of tyranny---but they are in fact experiencing a liberation. No longer denied access to a full range of movement, they feel strong, not helpless."

Please men or please ourselves? (Or does that have to be an or?) Defy or deny? Choice or coercion? Regular or decaf? Window or aisle?

Lately men, too, have been bombarded with media ideals, and they're actually starting to notice. "Lots of data suggest men increasingly worry about their bodies," says Judith Rodin. "They're targets for diet soft drink and light beer ads in a way we didn't see 15 years ago. The pressure is on both sexes, because the state of the body has become not merely a measure of fitness, but a metaphor for competence and a symbol of how willing we are to engage in self-corrective behavior." Physical flab is moral flab. Unfit means out of control.

I think it would be nice if we accepted that most of us aren't going to pose for the Victoria's Secret catalog unless Victoria's secret is that she ate a three-cylinder bag of Double Stuf Oreos.

I think it would be nice if we considered beautiful a range of women' s body types.

I think it would be nice if hating the way you look weren't so good for the economy. We all know that advertising is designed to make us feel dissatisfied with ourselves so that we'll buy more stuff (like the weightloss products on which Americans spend $30 billion a year). We know, too, that women in ads, knockouts to start with, are artificially perfected beyond human emulation. We know, but we forget. I think it would be nice if the next time you took a look at one of these ads and asked yourself What's she got that I haven't got? you answered yourself: an airbrush.

I think it would be nice if we brought back the bustle--for men. Nah. Really, I think it would be nice if all of us, men and women, were free to take whatever shape pleases us most--and to be sure it's us that we're pleasing.

Adapted from Health, copyright 1993. Used by permission.