Competition is cutthroat among Japan's thousands of pop-idol wannabes, but a unique concept is winning fame for a band of "chubby" girls deploying their cheeky cuteness to combat prejudices against obesity.

Despite one of the lowest rates of obesity in the developed world, Japan has a growing number of underweight young women, who rely on excessive dieting to satisfy society's emphasis on being slim.

Heavy people are the frequent target of jokes in the media and use of the derogatory debu, or "fatso", remains common.

Enter Pottya, a pop group named after a slang word for chubby and consisting of four young women who are not especially hefty to Western eyes, but well above average weight in Japan.

"People assume that being chubby is a sign of laziness or lack of self-discipline," says Michiko Ohashi, the group's heaviest member at 87 kilograms and a height of 1.67 meters.

"I became an idol with the hope of changing that image. If they see us working to make our dreams come true, we can show that chubby people can work hard."

First treated as a quirky joke at its debut in 2015, the group's image is changing, with the release of two albums and a growing roster of live concerts and television appearances.

Members range in weight from 63 kg to 87 kg for the 26-year-old Ohashi, and their average of 76 kg is about 26 kg over the Japanese average for women 13 to 18 years old, according to the Health Ministry.

Their body mass index, which compares weight to height, ranges from 27.4 to 31.2. Doctors consider an individual with a BMI above 25 to be overweight, and one above 31 obese.

"We're really heavy, but we want to use that to shake up the idol world," Ohashi says.

Pottya has passionate devotees, many of whom describe themselves as overweight.

Fans gather to meet Pottya members for a high-calorie lunch each month, with the menu of one recent meal featuring rice, noodles, chicken and potatoes - all fried.

The group was an inspiration, says Miho Kishi, who was bullied about her weight as a child.

"They've come out and are actually selling themselves as chubby, which has given us chubby women a lot of hope and courage," says the 25-year-old.

Michiko Ohashi performs with other members of Pottya at a fan meeting in Tokyo. Reuters