Matthew Yglesias has a graph showing percent of the population with a Body-Mass Indices >30% for 30 countries. (And, yes, I know that some NFL running back with 2.5% body fat would often show up as fat on this BMI index, but most people aren't NFL running backs, so it's a good enough measure for my purpose.)
The fattest country of the 30 is, of course, America. What's interesting is the second fattest country. We are constantly lectured that it is America's moral imperative to take in the hungry masses of Mexico, yet Mexico turns out to be the second fattest country out of the 30! In the U.S., Hispanics have fatter BMIs than whites, according to the federal government's NHANES study, so immigration from Mexico to the U.S. just makes them even fatter on average.