Overall, the numbers translate to 15 million adult Americans who are morbidly obese, defined as a body mass index of 40 or more.
In fact, the rate of severely obese is growing faster than those with moderate obesity, which has slowed.
The trend appears worse in women.
But the study found some good news. It said that the growth of the severely obese began to flatten out in 2005. So, while it's still growing, it's no longer growing as fast.
"The proportion of people at the high end of the weight scale continues to increase faster than any other group of obese people, despite increased public attention on the risks of obesity," explained Roland Sturm, lead author of the report and a senior economist for RAND. "But for the first time in the past 20 years there is evidence the trend is slowing."
The findings were also published online by the International Journal of Obesity.
According to the report, the trend affects all genders and ethnicities.
But severe obesity was 50 percent higher among women than among men, the study found. And, it was twice as high among blacks, when compared to Hispanics or whites.
To derive its result, RAND used data from the Behavioral Risk Factor Surveillance Survey, an annual survey conducted by the U.S. Centers for Disease Control and Prevention.
The survey tracks health risks in the United States and had more than 3 million, self-reporting respondents.