Percentage wise, are 9th. But look at the countries that are 1-8...
The population is 10,000 people in Nauru, 155,000 in Micronesia, 20,000 in Cook islands, 112,000 in Tonga, a whopping 1,398 in Niue, 188,000 in Samoa, 21,000 in Palau, and last but not least, 2.7 million in Kuwait.
And there's 300,000,000 (or 300 million) in the united states.
So you're right, we're not the fattest country percentage wise, but we're the home to the most fat people. And we're worse percentage wise then any compareable free western countries.
-EDIT-
I think I may as well reply to the other half of your post too. The government would not have to enforce excersize any more then private health insurance would. Since the public option would not be manditory. It'd be subject to its own premium, and not some direct tax like social security is. I don't see how a reasonable conclusion could be drawn that they would force people to stay in shape. (I could imagine them supporting it by offering some type of discount, or doing different things to help people get into shape since that'd help with the costs of the program.. but actually making it illegal to be unhealthy? Yeah right.)
As for your thoughts on BMI, I agree completly, its very out dated. Everybody is different.