The 'epidemic' in obesity isn't just from a redefinition of the BMI. There's something going on in the U.S. that is drastically affecting the average weight; no one seems to know what it is*. For example, just look at newsreel footage of crowds from a couple of decades ago. It's amazing how thin people seem.
Just a chance to explain my smoking theory
One theory of mine is people stopped smoking. Here is how it works, it is a known fact people who smoke when they stop gain weight. They replace the oral habitual habit of smoking with another of eating. The metabolism (and endoctrine system among others) is also altered when a person smokes, possibly due to the tobacco and the other stuff in a cig. Smoking also alters your appetite, to be less. Smoking can be a substitute for boredom eating, and snacking. You smoke rather than eat. Smoking was a huge part of the American culture for hundreds of years. In the last, say, 20 years we made a concerted effort to stop smoking, like the 1900's did with stopping chewing tobacco. Smoking is far more dangerous to the health than eating and thus became a substitute (better one in my opinion because smoking is highly toxic). I really don't suggest smoking for weight loss because it is so dangerous and if you stop you will gain all the weight back, if smoking doesn't kill you first.
There are other reasons of course why people gain weight and to point of obesity are things like genetics, disease and psychological reasons.
I think Aikido is a healthy lifestyle, the problem I see in Aikido regularly is the added calories from beer drinking.
I think we put too much stress on the "ideal" weight, and by doing so it does great psychological damage.