Overestimation of body weight among normal-weight adolescents is relatively uncommon; potentially a cause for celebration. However, almost half of boys and a third of girls with a BMI placing them in the overweight or obese BMI range perceived themselves to be about the right weight. Lack of awareness of excess weight among overweight and obese adolescents could be cause for concern.
It’s an open article in proof stage, you can read the whole thing: http://www.nature.com/ijo/journal/vaop/naam/pdf/ijo2015126a.pdf
TLDR: The fat kids thought they were normal, even too light, because fat acceptance.
It mentions ‘size acceptance’ once as a contradictory factor from healthy lifestyle programmes, chickening out in the Introduction, and keeps bringing up eating disorders unnecessarily, but it’s there, and fat boys are far more likely to think they’re normal.