One of the experts labelled the findings a "pile of rubbish" while another said it was a "horrific message" to put out.
The study, published in the Journal of the American Medical Association, suggested the overweight were less likely to die prematurely than people with a "healthy" weight.
Being underweight or severely obese did cut life expectancy, 'BBC News' reported.
Researchers at the US National Centre for Health Statistics looked at 97 studies involving nearly 2.9 million people to compare death rates with Body Mass Index (BMI) - a way of measuring obesity using a person's weight and height.
A healthy BMI is considered to be above 18.5 and below 25. However, overweight people (with a BMI between 25 and 30) were 6 per cent less likely to die early than those considered to have a healthy weight, the study reported.
Mildly obese people (BMI between 30 and 35) were no more likely to die prematurely than people with a healthy BMI.
The study said being "overweight was associated with significantly lower all-cause mortality".
Possible explanations included overweight people getting medical treatment, such as to control blood pressure, more quickly or the extra weight helping people survive being severely ill in hospital.
However, the researchers point out they looked only at deaths and not years spent free of ill-health.
"Have you ever seen a 100-year-old human being who is overweight? The answer is you probably haven't," Prof John Wass, vice-president of the Royal College of Physicians, said.
Wass said the largest people will have died years before and pointed to health problems and higher levels of Type 2 diabetes.
