Kim Stevens has a problem that affects tens of millions of Americans. If left untreated, it could lead to the death of someone she loves, someone who's part of the family.

Stevens' dog Dodger, a black and gray mixed breed, is obese. According to a new study, he's emblematic of a growing problem.

The majority of adult dogs and cats in U.S. homes are overweight or obese, and the problem has gotten worse over the past year, according to the Association for Pet Obesity Prevention. Fifty-three percent of adult dogs and 55% of cats were classified as overweight or obese by their veterinarians.

"I didn't notice the weight creeping on -- it was like all of a sudden he was just this fat dog," Stevens said as she and Dodger visited Atlanta's Centennial Olympic Park.

"His weight is about 82 pounds right now, and he should be 62 pounds." That means he needs to lose about a quarter of his weight -- equivalent to a 200-pound person needing to lose 50 pounds.

The reason is pretty simple: "Too much food and not enough exercise," Stevens said.

The Association for Pet Obesity Prevention report shows not only that more pets are overweight, but also that those with the problem "are getting fatter," said Ernie Ward, the group's founder.

The annual study, to be released next week and given in advance to CNN, found that 25% of cats and just more than 21% of dogs are obese. Both those figures are up slightly from 2010.

About 41 million dogs and 47 million cats are overweight or obese, the study found.

A long list of health dangers comes with the excess weight. "It's not a matter of if, it's when" serious complications will strike, said Ward.

These can include high blood pressure, "crippling arthritis," diabetes and some cancers. "Their life is shortened by two or 2½ years," said Ward, a veterinarian in North Carolina.

The U.S. Food and Drug Administration's Center for Veterinary Medicine, which cites the association's annual study, said the diseases seen among obese pets "are eerily similar to those reported for people."

A central part of the problem, the pet obesity group found, is the growing "fat pet gap:" More and more owners are unaware their pets are overweight. The study found that 22% of dog owners and 15% of cat owners characterized their pets as normal weight when the animals were actually overweight or obese.

"In simplest terms, we've made fat pets the new normal," said Ward.

In many cases, the problem correlates to the obesity epidemic among people, he said.

"This is the sentinel for childhood obesity. When I see dogs who are overweight, I see a child that's at risk for excess weight, because nobody's exercising. The kid's playing video games all day, the dog sits around all day," and "everybody's eating poorly."

Stevens said she needs to shed some weight, too.

"We'll do it together," she said.

The fact that obesity has struck her dog is particularly telling. Stevens does some work as a dog trainer. "To have a dog this heavy when you know better ... is embarrassing," she said.

But she has a lot of company. Thirty-nine percent of U.S. households own at least one dog, and 33% of households own at least one cat, according to the Humane Society of the United States.

The pet obesity survey was conducted at 41 veterinary clinics across the country and included evaluations of a sampling of more than 600 animals. "Over the five years studied, these results have proven to be consistent and increasing at a gradual pace," the association said.

While the obvious advice -- eat less, exercise more -- is the right starting point, there's more that pet owners should understand, Ward said.

Pet foods these days are "more calorically dense" than they used to be, yet owners are feeding their pets more, he said.

If you're concerned your pet may be obese, it's important to work with a veterinarian, and not try to tackle the problem on your own, said Ward. "Diet is not about starvation or deprivation. It's about gradual weight loss."

In many cases, carefully measuring food and committing to exercise can do the trick. But more severe cases need more extensive work.