A comparison of Great Britain household surveys from 1999 and 2007 show the number of people classed as clinically overweight or obese has increased.
But fewer adults now correctly class themselves as overweight, researchers report in the British Medical Journal. As more of us become obese, the average weight and average appearance has become heavier and rounder
In each of the surveys adults were asked to give their height and weight which was used to calculate their BMI.
And they were asked which clinical weight category - very underweight, underweight, about right, overweight or very overweight - they thought they fell into.
The proportion of obese people had nearly doubled from 11% in 1999 to 19% in 2007.
In 1999, 43% of the population had a BMI that put them in the overweight or obese range, of whom 81% correctly identified themselves as overweight.
But in 2007, 53% of the population had a BMI in the overweight or obese range, but only 75% of these correctly classed themselves as overweight.
The researchers from the Health Behaviour Research Centre at University College London said one reason for the findings could be that as a greater proportion of the population becomes overweight, people's perception of what is "normal" changes.
He added people needed to be made aware of how they measure themselves.