That theory could explain why the gap appears to be closing in what people say they weigh and what actual measurements report.
A new government telephone survey released Tuesday puts the adult obesity rate at nearly 27 percent and rising. A more scientific survey has already said the rate is 34 percent and holding steady.
Experts believe the 27 percent is probably an underestimate, because it's based on what people say. People tend to say they weigh less than they actually do and say they are taller than they are.
Nine states reported more than 30 percent of their residents are obese -- a far cry from 10 years ago when not one state had such a high prevalence of obesity, researchers at the U.S. Centers for Disease Control and Prevention say.
Not one state in the U.S. has met the national goal of lowering obesity prevalence to 15 percent, reported Dr. William Dietz, director of nutrition, physical activity, and obesity for the CDC, and colleagues.
Only one state -- Colorado -- and the District of Columbia reported a prevalence of obesity under 20 percent (18.6 percent and 19.7 percent, respectively), according to the CDC's second MMWR "Vital Signs" report on the issue.
"In 2007, only three states reported an increased prevalence of obesity above 30 percent -- Tennessee, Alabama, and Mississippi," Dietz said during a telephone press conference.
"Now, there are nine states that exceed [that mark]: Missouri, Kentucky, West Virginia, Tennessee, Arkansas, Oklahoma, Louisiana, Mississippi, and Alabama."
Mississippi had the highest prevalence, at 34.4 percent, and obesity estimates were ostensibly higher in the Midwest and South.
Dietz said that both surveys point out that obesity disproportionately affects certain groups.
The data found higher obesity rates among:
Non-Hispanic blacks: 36.8 percent
Hispanics: 30.7 percent
Individuals who didn't finish high school: 32.9 percent
Patients ages 50 to 59: 31.1 percent
Patients ages 60 to 69: 30.9 percent
The new results are based on a telephone survey of about 400,000 people who were asked their height and weight. CDC researchers then calculate whether the person is obese, following a standard formula for body mass index.
Under the formula, a 5-foot-4 woman is obese if she weighs 174 pounds or more, a 5-foot-10 man fits that description if he weighs at least 209 pounds.
The study found that nearly 27 percent of the surveyed adults said they were obese in 2009, up from about 25.5 percent in 2007, a small but statistically significant increase.
Earlier this year, the CDC released results from another study that actually weighed and measured 5,700 adults. It found that 34 percent are obese; results have been similar in the last three surveys.
The differing surveys mean the CDC is reporting that obesity is increasing - and that it's not.
"We have somewhat contradictory data," because the studies were done differently and sampled different populations, Dietz said.
The states were Alabama, Arkansas, Louisiana, Kentucky, Tennessee, Missouri, Oklahoma, West Virginia and Mississippi, the highest at 34 percent. In 2007, only Alabama, Mississippi and Tennessee topped 30 percent.
Washington's rate - just under 20 percent - could be tied to common use of public transportation there, or to higher rates of breast-feeding and fruit and vegetable consumption, he said.
CDC report: http://www.cdc.gov/vitalsigns