Unfortunately, many Mexican parents do not restrict what their children eat. Kids are allowed to eat whatever processed, sugary, salty, fatty food they can get, setting up a unhealthy foundation. Mexican food is also not very healthy, a lot of Beef, Pork, lard etc.. There are 2 types of Mexicans, those like my Pops who could eat whatever artery clogging food he wants and not gain a pound, and those like me who can't even look at some al pastor without gaing 10 pounds.
The thing about Mexican girls is, the ones that are skinny are that way because of their metabolism. They eat the same crap the fat ones eat. Eventually all those Takis and carne asada fries catches up to them, especially in their 20's. The ones that grew up playing sports like soccer tend to stay in shape though. I was near my old High School having lunch a week ago, and saw so many fat Mexican Girls it wasn't funny. But, I also saw a lot of in shape girls too.