THE STATE DINNER at the White House began with a mousse of shrimp in aspic, followed by a tenderloin of veal in a golden crust, a few stalks of buttered asparagus, then a delicate bibb lettuce salad. Dessert was a croquembouche of strawberries, washed down with champagne. One guest, a young woman with a healthy Midwestern appetite, grazed contently from her Reagan red plate.

Then she went home and had a ham sandwich.

Not that the food wasn't good. It was delicious. There just wasn't enough for a person who wasn't used to the delicate, teeny portions fed to America's rich.

Because the rich don't eat.

"I don't think anyone can eat and stay thin," says Buffy Cafritz, the socialite whose favorite cocktail is plain water with a lemon twist.

At Washington parties, never will you see Evangeline Bruce, the 126-pound, 5-foot-10 1/2 Georgetown hostess, cozying up to a groaning buffet table. Never will you see Ina Ginsburg, the 113-pound, 5-foot-4 socialite, with a chicken wing in her hand. And never will you see Nouha Alhegelan, the slim wife of the Saudi ambassador, eating heartily at dinner. In fact, at one luncheon last year, Alhegelan actually removed the skin from her trout.

"The more parties you go to, the less you eat," she says.

Zaftig doesn't work in this crowd. Nor can a size 12 Halston dazzle like a 6. Or check out the pages of "W," the fashion bible. Not only do the models have figures like grade school boys, but the exquisite pictures of chic food come in portions that might feed a gerbil. Better yet, check out Nancy Reagan. At the 1980 Republican convention, she chewed the same grape 32 times. It's an old dieting trick. "She eats," counters her press secretary, Sheila Tate. "She says she just worries it off."

"I never eat," says Steve Martindale, the lawyer and socialite. "I mean, I went to say goodbye to the Mexican ambassador last night, and I had one divine morsel, and then I said: 'I've got to get out of here.' Besides, you look like such a creep when you hang around the buffet table, filling your little plate with goodies. It looks like you never go anywhere."

Obviously, there is more to this than just calories. Although many of Washington's rich, social and powerful look as if they haven't had a square meal since 1975, the real reason they abstain has less to do with their waistlines and more to do with their hard-fought status in life. Not only is eating at buffets de'classe', it's what the uninvited do. "There are all these people in town who crash parties," reasons Martindale. "I figure they need to eat more than I do."

There's also the boredom factor. "When you've seen one shrimp, you've seen them all," says Tate, who sighs that she's put on 10 pounds in two years.

If you eat at a buffet, everyone will know you haven't been invited to a chic dinner afterward. And at the chic dinner? Never take seconds. It looks as if you haven't been invited to a chic dinner the next night. The menu rarely matters anyway. In fact, Washington social life has absolutely nothing to do with food. What matters is who comes to eat it.

"When we plan parties," says Jeff Ellis of Ridgewell's, the socially correct caterer, "one of the most important questions we ask is--'Who's coming?'" If it's the socialite crowd, Ellis knows to bring half the food he normally would. "When they entertain," he says, "smaller is better than bigger." As an example, he mentions the Meridian House Ball--an annual event where guests swish in late from embassy dinners. Consequently, Ellis never plans for as much food. "Now if that were another group," he says, "they'd be like vaccuum cleaners."

(Or like journalists. Ellis says that reporters are among the people in town who eat the most. It's a collective appetite that travels well, too. At the economic summit at Versailles in June, 2,000 journalists and technicians, eating free in the palace's Orangerie, inexplicably consumed--in the first two days--10,000 lunches. That included 1,500 pounds of salad and hundreds of pounds of cold cuts. The French caterers had to scurry back to Paris for more.)

Interestingly, many on Washington's social circuit insist that they do, in fact, eat. They say it's the others who don't. Joan Braden, the public relations executive and party-giver, reports that Evangeline Bruce eats nothing. But Bruce insists she's always the first to dig in and the last to finish. "I'm famous for it," she says. Martindale says Cafritz hasn't eaten in years, but Cafritz reports that it's really Ina Ginsburg who has the willpower. "She eats very sensibly," says Cafritz. "I just saw her. She had a little sandwich, a tiny little sandwich." As for Helga Orfila, the former fashion model who's married to the secretary-general of the Organization of American States, everybody says she never eats at all.

"I eat all the time," she responds. "I'm not as thin as I used to be."

Well, what does she weigh?

"No, no," she says. "It's disgusting."

Then there's the "no-problems" school, comprised of party-goers who insist they eat whatever they want. Although it may be a single radish, the important thing is to make it clear that keeping svelte is an effortless process.

"I think I'm just naturally thin," says Joan Braden.

"I don't seem to have a problem," says Ginsburg.

Others engage in competitive non-eating. This often occurs in restaurants. One diner will order broiled fish, only to be one-upped by a companion diner who orders the same slab of fish--without butter. The only revenge is watching the unbuttered fish arrive, parched and pathetic.

Then there are those who don't care at all. "I'm usually the one eating rolls and butter when everyone else is eating broiled fish," says Oatsie Charles, the Georgetown doyenne. Asked if she tries to restrain herself at parties, Charles replies: "Darling, have you ever seen me? If I go to a party, I usually try to enjoy myself."

Because in Washington, it's tough to be skinny. "I can refuse things like potatoes," says Jean Smith, the wife of the attorney general, "but I never ate bread until I got to Washington. I just don't know why I do it." She goes out an average of three nights a week and has gained eight pounds in two years. But she won't say exactly what she weighs. "I don't want my husband to know," she sighs.

Three hours later, Smith is calling back. "I just wanted you to know," she says, "that after we talked, I got on my exer-cycle and then weighed myself. I lost three pounds.

"Which means," she says brightly, "that I've only gained five."