TOKYO, JAN. 22 -- Here's another stereotype about Japan you can toss into history's trash can: Those "little Japanese" aren't so little anymore.

The rapid westernization of this traditional Asian society has brought such major changes in diet and lifestyle that Japanese people have experienced one of the fastest collective growth spurts ever recorded, according to public health officials.

In the last 30 years, the height of the average Japanese male has gone up nearly four inches, while average female height has increased about 2.7 inches. The average 20-year-old Japanese man today is 5 feet 8 1/4 inches, according to data released this month by Japan's Health and Welfare Ministry. That is about the same height as European 20-year-olds.

The last U.S. survey, in 1976-80, found 20-year-old males averaged 5 feet 9.7, an inch taller than the figure for 20 years earlier.

If today's Japanese 10-year-olds continue the current growth pattern, as is likely, they will be just about as tall and weigh just about as much as their American peers when they graduate from high school in 2000, health officials here say.

Public health experts cite several reasons for this dizzying change in the stature of an entire nation. But if you had to explain the transformation in a single word, it would probably be hamburgers.

If it is true that you are what you eat, the Japanese are much more westernized people than most Americans may realize.

The fundamental change in the Japanese diet is visible every day at noon in every Japanese city, as people turn away from the traditional lunch of rice balls wrapped in sea weed to form long lines in front of places like McDonald's and Kentucky Fried Chicken, which now rank as the No. 1 and No. 2 most popular restaurants in Japan.

On residential streets every evening, the familiar delivery men carrying traditional rice and noodle dishes are being shouldered aside by fleets of bright red motorcycles bearing names like Domino's, Shakey's, Pizza-La and Trump's Pizza and Tacos.

"The chief reason for the increase in body size is almost certainly diet," said Nobumichi Sakai, director of the Nutrition Branch at the Health and Welfare Ministry. "The dominant pattern of Japanese dietary change since World War II has been westernization. Grains, particularly rice, have declined in importance, and the caloric intake from animal foods has increased sharply. Meat and dairy consumption has gone way up."

"This is one of the mysteries of Japan," Sakai continued. "Once we decide to do something, all over the country, everybody does it."

In addition to obvious differences -- people on the street are taller and bigger -- the new Japanese diet is changing in public health patterns. Officials can now see the beginning of an obesity problem among children, although the figures are far lower than in the West. Some forms of cancer previously unseen here are becoming more common.

However, the increase in meat consumption has not led to serious problems with heart disease, according to the Health Ministry. Japanese fat intake is about 25 percent of total calories, as opposed to 35 percent in the United States. "One of the American government's policy goals now," health official Sakai said proudly, "is to reduce fat consumption and get closer to Japanese levels."

Another factor making the Japanese taller, according to the Health Ministry, is that Japanese people today commonly sit in Western-style chairs at home and work, instead of kneeling on rice-straw mats as they did for centuries. "This has ended the constant pressure on the knees of Japanese children, and they grow up to be taller people," explained Masatoshi Hara of the Health Ministry.

Some U.S. experts on Japan argue that this ancient society is highly resistant to change. Others maintain that Japan is becoming more like America all the time. Dietary trends here support the latter view.

Anybody who thinks Japan can't change should visit the Minamidai dorm at Nippon University, which houses the school's championship team in the sport of amefuto, or "American football." The hulking tackles and linebackers look pretty much like American football players -- and they eat like them too.

"These kids eat six meals a day -- if you call a couple of hamburgers and fries a meal," said Yuko Miyajima, the team manager. "They show up here with their school stuff in one hand and a Big Mac in the other."

There is still room for improvement in football skills here, though. In last month's Epson Bowl game, when a Japanese college all-star team played American Ivy Leaguers -- generally the doormats of the U.S. sport -- the final score was Ivy League 68, Japan 3.

The change in Japan's food consumption has led to an equally historic change in food production. The Agriculture Ministry's latest annual report shows that meat and dairy products have replaced rice as the chief cash crop here.

Japan has been a rice-dependent society for about 1,000 years. Rice planting and harvest rituals are central to the country's culture and to the indigenous religion, Shinto. But in the last quarter-century, per-capita rice consumption has fallen nearly 50 percent, according to government data. Far more than other nations of the Asian "rice bowl" culture, Japan has switched to westernized menus.

Virtually every Japanese home has an electric rice cooker, and most housewives still make a pot every morning. But more and more, the rice is a supplement to meals of meat, vegetables, bread and margarine that would look familiar on an American table.

The traditional Japanese breakfast -- rice and seaweed, pickled vegetables, and soybean-paste soup -- has been replaced almost everywhere by a "morning set": toast, cereal, scrambled eggs and tossed salad (a breakfast item people here think they learned from America). This is so common that some hotels advertise that they offer a Japanese breakfast in addition to the expected Western version.

The new farm production data could impair the Japanese government's campaign to continue its nearly total ban on imports of rice. Despite enormous pressure in the current round of international trade talks, Japan has been fighting to keep its rice market closed. The government argues this is needed to ensure domestic production. The new data suggest, though, that food imports actually increase domestic production.

The surge in meat and dairy output in the 1991 figures came after the government liberalized meat imports. Farm interests here had warned that permitting meat imports would destroy the high-cost domestic meat industry. Instead, the government study shows, lower-priced imports increased overall demand, and turned out to be a boon for Japan's meat producers.

Some economists here argue that the same phenomenon could occur if low-price foreign rice could be sold here. "This news may be ticklish for the Japanese argument against liberalizing rice," the financial newspaper Nihon Keizai Shimbun noted recently.

One reason for rice's fall from its traditional No. 1 position is that farmers, foreseeing an eventual end to the import ban, are moving into other cash crops. Not only meat and dairy but also fruits, vegetables and mushrooms are on the increase, while rice production remains flat.

This sharp change, in turn, has sparked a countermove, as rice interests adopt familiar Western promotional ideas to sell their product.

Playing upon the increasing Japanese fad for diets, national agricultural groups built big campaigns around a popular book by food writer Sonoko Suzuki titled, "Get Slim by Eating Rice."

A key marketing target for rice growers is the industry that did them in in the first place: fast food.

"The obvious fact of Japanese eating trends is of course westernization," said Joel Silverstein, a vice president of Kentucky Fried Chicken Japan. "But rice is making a comeback, and restaurants like ours are going to have to do something about it."

Even McDonald's has introduced seaweed-wrapped rice balls and curried rice in its Japanese shops alongside the burgers.