Why are northern Mexicans 6 inches taller than southern Mexicans?

Answer 1

Due to food and racial mix.

Northern Mexico has better grounds for livestock feeding, so at least until the 1950's when electricity and refrigeration became more common, many more people in the northern states ate fresh meat and milk products while the southern, more tropical states, have poorer soils where only pork and chicken were common. This food difference is seen up to this day, as meat products are more expensive in the southern states.

The other cause is racial mix. When Spaniards conquered central and northern Mexico in the 16th century, intermixing with the native population became common, so many Caucasian traits (including eye color, baldness or height) were diffused into the population. However, the Mayas were not fully conquered until the late 17th century, and even then racial intermixing was rare. The Maya characteristics are more common on the southern states, specially on the Yucatan peninsula.