University of Cambridge researchers said about 676,000 deaths each year were down to inactivity, compared with 337,000 from carrying too much weight.
They concluded that getting everyone to do at least 20 minutes of brisk walking a day would have substantial benefits.
Experts said exercise was beneficial for people of any weight.
Obesity and inactivity often go hand in hand.
However, it is known that thin people have a higher risk of health problems if they are inactive. And obese people who exercise are in better health than those that do not. BBC News