These false impressions were more common in black and Mexican-American children than in white kids, the survey found.
Some experts say such findings are reason for concern because obese children who permanently settle into an unhealthy physique face diabetes and other health problems later in life.
Studies have repeatedly shown many adults — especially men — think themselves taller and thinner than they actually are. And an earlier study of U.S. adolescents found nearly a third misperceive their weight. But the new report is noteworthy because it comes from a gold-standard federal survey that not only interviews kids, but also weighs and measures them.
The survey is "a true snapshot of what the U.S. population thinks their weight status is," said Kendrin Sonneville, a Harvard researcher not involved in the study.
The Centers for Disease Control and Prevention did the research, and released the results Wednesday. Researchers focused on the responses of more than 6,100 children and adolescents, ages 8 through 15, who participated in the survey during 2005 through 2012.
—About 30 per cent misperceived their weight status, and a large majority were heavier kids who thought they were normal weight.
—Nearly half of obese boys and more than a third of obese girls consider themselves to be about the right weight.
—About 34 per cent of black and Mexican-American kids misperceived how heavy they were. About 28 per cent of white kids did.
—This kind of misconception was more common among children in lower-income families than those in households with more money.
The findings echo what's been seen in adults. An earlier CDC study — drawing on the same CDC survey — found nearly 40 per cent of overweight adults and about 8 per cent of obese adults considered themselves to be "about the right weight." That study also found weight misperceptions were more common in blacks and Mexican-Americans.
Earlier studies have also found that among overweight women, blacks tend to be more satisfied with their bodies than whites.
It's possible children are adopting their parents' standards, one of the study's authors suggested.
"It could be they look like other people in the family who don't perceive themselves as overweight," said the CDC researcher, Vicki Burt.
If children understand their weight, it can help them to take action and head off future health problems, said the study's lead author, Neda Sarafrazi.
But there's some debate about the impact of children judging themselves fat. One of Sonneville's studies found that overweight adolescent girls who liked their bodies were less likely to binge eat and less likely to gain weight over time than overweight girls who didn't like their bodies.
"On the surface, many people assume weight misperception is a bad thing," Sonneville said. "But it remains to be seen if it is harmful or helpful."
CDC report: http://www.cdc.gov/nchs/