His Majesty King Taufa'ahau Tupou IV of Tonga is a man of many interests. He's an amateur archeologist and history buff with a law degree from the University of Sydney. He's an accomplished sportsman, a surfer and skin diver who holds the Tongan junior-grade pole-vaulting record. And he's the author of a music textbook who plays the piano, saxophone, clarinet, guitar, balalaika and, occasionally, the Tongan nose flute.

"People who are not familiar with the king," says Rep. Fred Eckert (R-N.Y.), former U.S. ambassador to Fiji, not far from Tonga in the South Pacific, "are normally quite surprised by his breadth of knowledge."

They are also, truth be told, quite surprised by his breadth.

The king, who has been in town all week on a private visit -- attending Thursday's Presidential Prayer Breakfast, paying calls on old friends like Eckert and new ones like Vice President George Bush, and, he adds, "going to lots of lunches and dinners" -- is listed in the Guinness Book of World Records as the world's heaviest monarch.

In September 1976, the Guinness people report, the 6-foot-3 king "was weighed on the only adequate scales in the country at the airport recording 462 lb."

To all that, the king of Tonga smiles and shakes his head.

"I have been on a diet to reduce my weight," he said the other day during a wide-ranging interview in his suite at the Washington Hilton, "so I'm not at all as heavy I used to be. I find I can control my weight quite easily just by eating a lot of soup. I would say that I'm down to, oh, about 360 pounds."

The king, visiting Washington for the third time, is a genial, unpretentious sort of monarch. He greets his visitors -- once his private secretary has screened them, the Secret Service has checked them and his aide-de-camp has ushered them into the royal presence -- with a warm, enveloping handshake.

He stands impressively in the middle of the room, squinting through heavy-framed glasses and breathing audibly, and addresses his aide in what sounds like brisk Tongan. The aide replies in rather respectful Tongan, bows slightly and backs away.

"Please sit down," says the king. He positions himself behind a low table dominated by a tray of fruit. He backs up to the sofa. After a while he is seated.

He wears a blue double-breasted suit, custom-made in Singapore, and, suspended across his chest from shirt pocket to shirt pocket, a massive chain. There is, he later reveals, a commemorative 1984 Olympics pocket watch on one end, a Swiss army knife on the other. He uses the pocket watch in tandem with his wristwatch to keep track of time zones on his far-flung travels. "I find the knife very useful for many things," he adds, but politely declines to be drawn into elaboration.

"The prayer breakfast was very good," he says of his morning meal. "There were eggs and some sort of piece of steak, but they provided no Tabasco sauce. I like to put it on my eggs. Luckily, I brought my own little bottle. I like to take a bottle of Tabasco sauce with me wherever I go."

He produces the bottle and shows it off.

"But I had to leave the prayer breakfast halfway through," the king continues. "I take high-blood-pressure tablets, and they make you want to go to the bathroom. But they are also a good excuse to leave early without seeming to be rude. It is a very convenient way to get away."

Chuckling, he says he managed to get away longer by taking lunch in his room. "I had a Big Mac," he says, and chuckles some more.

His kingdom, a string of 150 islands in the South Pacific, is tiny. His 100,000 loyal subjects, by and large, are huge. Big is beautiful, according to Polynesian tradition -- and so Tongans eat as much as possible to stay that way.

"They consume an amazing amount of food," says Bill Granger of the Agency for International Development, who spent five years in Tonga with the Peace Corps. "Fish, lobster, yams, taro root, tapioca root and ufi, which is a white potato. I've seen ufi that were six feet long."

The king is slightly graying at 66. He was crowned on his 49th birthday, July 4, 1967, an event for which 40,000 of his people gathered for five days of celebration in the Tongan capital of Nuku'alofa, where they roasted 71,000 suckling pigs, drank oceans of kava (a pepper root drink that causes pleasurable numbness), sang songs and danced dances.

"The distinguishing characteristics of Tongan dance," says the Encyclopaedia Britannica, "are the emphasis on the rotation of the lower arm and the flexion and extension of the wrist, as well as a quick sideward tilt of the head. The legs are used mainly to keep time with sideward stepping movements, and there is a marked absence of hip or torso movement."

The king -- who can trace his lineage to King Siaosi Tupou I of the early 19th century and before that to the Tu'i Tonga, the sacred kings of the 10th century -- did not himself dance or sing. "I was a spectator," he says.

"The people of Tonga can sing songs all night long without repeating themselves -- and frequently do," he adds.

At the time of his coronation, he weighed in at a diminutive 294 pounds. He has since grown into the job.

"I very much enjoy the singing of hymns," says the king, a Methodist like most of his subjects. "But I must say, until I arrived in America, I never knew that 'Amazing Grace' could be sung so many times in one day."

His kingdom, a former British protectorate dubbed the "Friendly Islands" by Capt. James Cook in 1773, is these days a favorite port for ships of the U.S. Navy. Having won its independence from Britain in 1970, it is now a constitutional monarchy in which the king's younger brother, Prince Tu'ipezehake, serves as prime minister.

The kingdom's gross national product of $30 million is based largely on agriculture (passion fruit, yams), but there is also light manufacturing (sweaters, soccer balls) and, says the king, the beginnings of oil exploration.

"We actually have crude oil seeping up into the water table," he says. "The oil is very similar to oil in Texas. Various companies have started drilling."

The king, who had separate meetings with Bush and Eckert on Wednesday and briefly crossed paths with President Reagan at Thursday's prayer breakfast, says he got to know Nancy Reagan in 1981 at the wedding of Prince Charles and Lady Diana (for which the British built him a special chair).

"I know Margaret Thatcher quite well, and also her husband," he says. "It's her husband's job to be the timekeeper. Whenever it's time for her to go, he begins banging his spoon on the table."

He says he leaves this evening for London, then on to Spain. The royal entourage includes Her Majesty Queen Mata'aho, the queen's lady-in-waiting, the king's aide-de-camp, a school-age girl whom the king describes only as a member of the royal family, the king's private physician and various other aides.

He smiles and shifts his weight on the sofa. He smiles some more.

"I have given many interviews to the Tonga Chronicle and also on the government-owned radio station, but this is the longest interview I have ever given," he says good-naturedly.

But there's always time to talk about the weather.

"I have seen snow many times, but for some of the people with me, this is their first time," the king says. "I think they are rather surprised by how light and feathery snow is. I think they expected it to be hard and icy, like hail. We sometimes have hail in Tonga."

As for the king, "I have no trouble dealing with cold weather." He bends over to point out his fleece-lined leather boots. He lifts up a pants leg to reveal his long underwear.

Then the aide-de-camp is dispatched to the bedroom (where the king-size bed has been specially reinforced), from which he returns with a broad sheepskin coat, made for the king in New Zealand. The aide holds it up for inspection.

"You won't see another coat like this anywhere," says the king. "Feel how soft it is," he urges. The heater in the room is going full blast.

"Wherever I travel," he says, "I come prepared."