Their acknowledgement will no doubt be welcome news for the likes of cook Nigella Lawson and singer Adele, who have long argued it is possible to be fat and fit.
Writing in a medical journal published by the respected Lancet group, the medics said accurately identifying those who are fat and fit would cut the bill for obesity treatment.
Targeting treatment to those who would benefit the most could ensure taxpayers' money is best spent.
"Potentially scarce resources can be more effectively tailored?...?Some prevention and treatment strategies can be very expensive and time-consuming," the researchers said.
Genes are thought to be key, but working out why some of our bodies resist the toll of excess weight could also lead to treatments to improve the health of the obese who are not fortunate enough to fall into the healthy category. Doctors normally gauge whether a patient's size is a health concern by using their weight and height to calculate their body mass index, or BMI.
But relying on BMI is controversial, as it does not distinguish between muscle and fat, meaning some athletes are classified as obese.
The German researchers said using BMI alone was "insufficient" and other factors such as waist size, blood pressure and where fat is stored in the body needed to be taken into account.
But much more research is needed before any formula is ready for use in doctor's surgeries. And in the meantime, the researchers say that the obese should still think about losing weight.
The team from the German Institute of Human Nutrition and the University of Tubingen said: "Prevention of obesity through healthy diet and physical activity should be widely promoted."
Writing in the Lancet Diabetes & Endocrinology journal, they warned that not everyone who is "healthy obese" will stay that way.
Dr Ian Campbell, a GP and medical director of charity Weight Concern, said it made "perfect sense" that some people could be fat but healthy.
But he warned: "The science is not yet precise. There is always a risk that you will develop problems later."
- DAILY MAIL