You answered your own question. The mexican diet is loaded with calories and saturated fats when they can indulge in them. In an environment where they are constantly working, mostly limited to water and can't afford to have too many servings it is 'okay'. Like southern cooking or soul food its optimized for a poverty stricken lifestyle (still not healthy).
Now imagine what happens when you place these same people and diet in a first world country. Due to culture and inculcation they don't suddenly change their diet. Now they're not working nor fasting quite as much, they're able to get access to things like tortillas, pastries, carbonated drinks and in much higher servings than necessary. The wife has access to appliances in a modernized country so the homemaker role is made much less strenuous. Hence they gain weight. Their culture says to enjoy food, not realizing the original reason for this was because of the harsh environment (ie. they might not know when they get their next meal). Plus mexican women have no incentive to stay thin. Its not like the husband will leave them. Plus divorce laws are in their favor, like all western women.
Note Mexican men tend to be less fat because they are usually out wageslaving so much or Juegan futbol.
