I think it might not just be a reference to weight.. Some Americans are rowdy and might start to jump on those bridges and causing it to be unstable...
That would be nations- race is a different sort of category, there are multiple races in America itself. This would be more of a nationalistic prejudice. I think this was an unfortunate translation error, but it does play into the stereotype that America is overweight, I agree
But most Americans I know who travel are in pretty good shape. I don't think many highly overweight people would enjoy expending that much energy when they could just relax at home.
Please stop saying that Americans are fat. Not all of us are.