According to 2006 figures released by food service juggernaut Aramark, the average adult in the United States has a Body Mass Index of 29.
(Note: A healthy BMI ranges from 18.5 to 25. 'Overweight' is characterized as 25 - 30, and obese is marked by a BMI of 30 or more).
As I have mentioned in the past, BMI (essentially a weight: height ratio) is not the most accurate measurement of weight status when dealing with individuals.
One limitation to this formula is that it does not differentiate between muscle and fat.
Therefore, a bodybuilder will misleadingly have a BMI in the "obese" category.
When looking at large populations, though, I find BMI to be an accurate barometer, particularly when we are talking about the average adult in this country being on the verge of clinical obesity.
It is also worth pointing out that there is a clear upward trend.
According to the Centers for Disease Control and Prevention, the average adult in the United States had a BMI of 25 in 1960.
I very, very highly doubt that the latest figure of 29 is due to more muscle mass, especially since this perfectly corelates to the increasing amount of calories consumed per capita in the past forty years.