We in the United States shouldn't be surprised. Sure, I could shed a pound or 50. But who has the time and motivation. If someone asked me to travel 100 mile in the hot sun with little or no food for three or four days, I would have to say, "You go ahead. I'll catch up with you later. In fact, why don't I stay here and mind the food that you're leaving behind."
Traversing the US–Mexican border is not for the faint of heart. While you don't have to be a triathlete, you do have to give up a several servings of donuts and put in a several days of cardio.
So we're dealing with simple math. As the more motivated, athletically inclined Mexicans moved north and their fluffier counterparts stayed behind, Mexico became a weightier nation. Hurray for our side.
Will our hefty Hispanic neighbors ever migrate this way? Not if Wal-Mart is any indication. Have you seen our large North-American associates get around the aisles in those electric shopping carts? "Run for the border" would be more of a motorized crawl. Not much of a chase for the Border patrol (as long as they're in shape).
Don't worry. Mexico's achievement is temporary. The ambitious will continue their momentum and travel north into Canadian lands. The really ambitious will overshoot Canada, skidded pass the north pole, and stop their rushin' in Russia. The number of skinny folks in the US will drop and we will regain our title as "Fattest Country of the Solar System".
USA! USA! USA!