PHILADELPHIA — It was no Weight Watchers meeting.

Under the glittering chandeliers at New York's famous Tavern on the Green restaurant, Thunder, formerly known as the "black flabrador retriever," padded up to the scale. His competition: a miniature pinscher, a beagle, an English Lab, and a Chesapeake Bay retriever.

Starting off at 178.8 pounds, the question was, would Thunder be the biggest loser in the canine slim-down competition?

Thunder was always mighty big, but when his owner, Linda Leigh Sacco of Abescon, N.J., got busy with two jobs, he became mighty fat.

"He never ate a lot. He just didn't get a lot of exercise," Sacco said. "He's just a larger-boned breed."

People know that childhood and adult obesity pose health and social problems. But on animals, well, come on, rub that soft belly! Look at that cute waddle! Give that cuddly muffin a treat!

Except that it's not so cute anymore. With 40 percent of animals over the age of 4 regarded as overweight, veterinarians say they are seeing more diabetes, joint problems and other health issues similar to those of their heavy humans. And they are calling on owners to step up to the plate, so to speak, and put their pets on a diet.

"There really is an obesity epidemic," said Christine Bohn, medical director of VCA Cat Hospital in Philadelphia. "As a profession, we're recognizing it more and more, and it's preventable."

In the interests of full disclosure, Bohn admitted that both her cats, Simon and Stanley, are dieting. They had shed pounds chasing her laser pointer but gained it right back after Bohn went on vacation.

"My friend just put down food, and they ate as much as they wanted," she said, laughing. "They gained two pounds back — each."

Vets say pet fat is not totally the fault of people. More animals are spayed and neutered, slowing metabolism and increasing obesity. Animals tend to stay indoors more, particularly in urban areas. Apartments may be safer, but cats burn more calories climbing trees than couches.

Not everything can be blamed on environment, said Kathryn Michel, an associate professor of nutrition at the University of Pennsylvania School of Veterinary Medicine.

"The largest growth in the pet industry is the treat division," Michel said. "There didn't use to be cat treats."

Last year, she studied the success of the "CatKins" all-protein diet. It worked just as it does in people.

"The bottom line is that if they had their calorie intake restricted, they lost weight," said Michel, who is now studying whether dieting dogs are more aggressive.

"Those on low carbs and low calories lost exactly the same amount of weight."

Sounds easy, right? You control the food and the exercise. You're the boss. But even Jane Brydon, a dog trainer, is powerless over her pooch.

Brydon said Gracie does a "knockout" rollover and never jumps on visitors. But when it comes to snarfing the cat food, even Brydon cannot prevent "counter-surfing."

"I have a baby gate that blocks off the kitchen from the living room, and now I have a rocking chair blocking the baby gate," said Brydon, whose Jack Russell-Chihuahua mix weighs 12 pounds, instead of the optimum nine.

Even people who motivate human clients to climb mountains and drop pounds melt in the face of, well, that face. The one with the liquid eyes and the begging and the pet-my-belly plea.

Jillian Michaels, the tough personal trainer who bullied contestants on the weight-loss show "The Biggest Loser," was a complete softie when it came to her Chihuahua, Baxter.

"I just thought it was cute and he's a dog, who cares?" she said.

There were meatballs. Scraps. Accidental treats, like the entire box of hard candy her friend left on the kitchen table. Suddenly, he was 12 pounds.

When her vet told Michaels that her dog could live an extra five years if he shed some weight, the trainer got motivated.

Now instead of meatballs, he gets Science Diet treats.

"It's like methadone to a heroin addict," she said.

Michaels was one of the judges at the recent National PetFit Challenge, sponsored by Hill's Pet Nutrition, Inc. With the Tavern on the Green's normally tourist-filled hallway covered in cat litter boxes, and a terrace available for canine relief, Baxter's final weigh-in was a success — down to 8.6 pounds.

Thunder also proved to be an excellent example, dropping 44.4 pounds to an athletic 134.4 with the help of daily walks and low-carb, high-fiber pet food.

Alas, he wasn't the biggest loser; Milo, the miniature pinscher, stole the crown after dropping from 22.8 pounds to 10.6.

Thunder took it in stride. He padded back outside, and relieved himself.

"Now he knows when it's time to go walking," his owner said. "Sometimes he brings the leash, but sometimes he brings his dog bowl."

Old habits die hard.