Yes, that all-American diet of refried beans with lard and tortillas soaked in lard is what is doing it.
unfortunately for the mexicans, its the perfect storm of fattening Gringo food like Pizza, kielbasa, Gyros and fried Dim-Sum along with the *really* fattening Gringo food like cheese covered nachos, refried beans, burritos and huevos rancheros...
Not to mention our sedentary lifestyle. You know all those siesta's of ours that the Mexican's have picked up on.