The optimum ratio of weight and height -- known as body mass index or BMI -- is now on the upper side of the healthy range, according to the report in the Journal of the American Medical Association (JAMA).
"Compared to the 1970s, today's overweight individuals have lower mortality than so-called normal weight individuals," said Børge Nordestgaard, clinical professor at the University of Copenhagen and Copenhagen University Hospital. "The reason for this change is unknown."
The report was based on more than 100,000 people in Denmark. The study spanned three groups, or cohorts, whose risk of dying for any reason was examined in 1976-78, 1991-1994, and in 2003-2013.
Currently, doctors define the normal range for BMI -- calculated by weight in kilograms divided by height in meters squared -- as between 18.5 and 24.9.
A BMI of 25-29.9 is considered overweight, while 30 or higher is obese.
In the 1970s, the optimal BMI for the lowest risk of death was 23.7.
This would be the equivalent of a 1.83 meter (six-foot) tall man who weighs 77 kilograms (170 pounds), or a 1.65 meter (five-foot-five) woman who weighs 65 kilograms (143 pounds).
By 1991-94, the optimal BMI had risen to 24.6. And in 2003-2013, it reached 27.
Compared to four decades ago, that would mean adding 14kg (30 pounds) to the frame of the a person who stands six-feet tall, or nine kg (19 pounds) to the frame of someone 1.65 metres tall.
Researchers also found that obese people in the 1970s were more likely to die than normal weight people, but this association disappeared in the 2000s.
"The increased risk of all-cause mortality associated with obesity compared to normal weight decreased from 30 percent 1976-78 to 0 percent in 2003-13," said principal investigator Shoaib Afzal, of Copenhagen University Hospital in Denmark.
Health authorities have long warned of the risks of being overweight, including diabetes and cardiovascular disease.
Nordestgaard said the current findings suggest "a need to revise the categories presently used to define overweight, which are based on data from before the 1990s."
However, experts cautioned that the biological mechanism behind the new findings remains poorly understood, and the results should not be interpreted to mean that people can cease caring about what they eat.
According to Naveed Sattar, professor of metabolic medicine at the University of Glasgow, the study is "of interest" but dos not merit changing advice on obesity and preventing extra pounds.
"In recent years, as populations become more obese and with wider availability of cheap preventative medications many more such individuals are likely to be better treated for abnormal blood pressure, abnormal cholesterol and, if also present, type 2 diabetes, leading in turn to lower death risks," said Sattar, who was not involved in the study.
"In other words, the current findings do not mean that being overweight is protecting you from death, far from it -- rather, many confounding factors may give the current result and we know from many other studies that being overweight or obese does increase mortality risks, in the same way that it
increases risk for many other conditions."