Sadly I can't say, because I can't get the full text of the paper that everyone points at when they say over half of Americans are overweight, at least not without paying for it (anyone with academic access to scientific papers willing to see if they can get a full copy for free?). All I can say is that the median BMI is 25.5. However, given the percentage of Americans with a BMI over 25 (59.4% of men, 50.7% of women) and that median number, I'm pretty darn sure more than half of Americans are below a BMI of 27.