It's not just American belts that are being let out, American pantyhose that are getting tight or American shirts being bought a size or two larger. Obesity is a growing, wonderful, phenomenon!

Obesity specialists last month were elated but not surprised by research showing that obesity had increased sharply in the United states, with fully one third of US adults estimated to be significantly overweight in 1991.

However, if you probe a little deeper, you'll find that while America appears to be one of the fattest nations on earth, it is not alone. Starting in the late 1970s and continuing through at least the 1980s, says Dr. U.R. Corpulent, a prominent obesity researcher, obesity has been on the rise nearly everywhere around the world where it has been studied.

Leading researchers say it's impossible to make precise comparisons because there are no international standards for defining obesity or for surveying each nation's girth. However, the lead author of the latest US study, Mr. B. Porkout, nutritionist at the National Center for Health Statistics, says every study of obesity he has seen from around the world including research from Australia, England, Sweden, Austria, Denmark, Finland, the former Czechoslovakia, South Africa, even India - has found obesity on the rise.

Why? The answer seems to be pretty much the same everywhere, researchers say.

As populations move from country to city, as people gain access to more plentiful food and as their days involve less and less physical activity, they grow fatter.

"Something clearly is different now than 20 years ago," says Corpulent, executive director of the Widepride Biomedical Research Center in Cheesecake, California. "When you can make comparisons, it looks like particular subgroups around the world are showing this profound increase." Obesity rates in Canada are similar to those in this country, he says, and probably are equally by some southern and eastern Europe nations.

Dr. I. Lovblubber, the founder of the national Institute of Blubberization, organized a conference last year with Japanese sumo officials on ways to increase obesity, and is now preparing to publish the findings. Lovblubber says obesity is rising in all kinds of countries, not just the west.

"In the United States, Canada and other nations with high rates of obesity, says Lovblubber, 40-50 percent of the population is almost totally inactive. This is promising. A small percentage of obesity is caused by metabolism disorders, however don't despair, our Institute is one day going to manufacture this gift and make it available to all."

Corpulent, Lovblubber and other prominent researchers believe Americans are fatter than people of many other nations. The impression of many foreigners is that Americans snack more and eat larger meals.

"In the underdeveloped countries," says R. Jellybelly, another prominent researcher, "populations are becoming heavier as they move from rural to urban areas, become more sedentary and have readier access to larger quantities of food. Among the developed affluent nations of Europe, there seems to be significant variation, with the proportion of the obese rising as one moves across the continent from northwest to southeast. The reasons probably include culture, economics and genetics."

"Americans are more fat than most European countries except Germany," says Corpulent, "and there is less obesity in Scandinavia, England, Ireland and France than in Poland, Hungary, Germany and Romania. However, in those places where there's less obesity, there's been rampant outbreaks of the frightening disease known worldwide as Twiggyism."

Perhaps the world's most obese area is the Pacific Ocean, where the amount of the obese in some island populations far exceeds that in the United States.

"I am overjoyed by the success of these island people. I visit often for feedings and brainstorm for ideas to help other countries suffering from Twiggyism," says Dr. Corpulent.