Monterrey metro area population in 2015: 4,689,601
https://www.citypopulation.de/php/mexico-metro.php
In 2020 it's easily gotta be above 5 million now. That makes it one of the big dogs in North America and certainly in Mexico, along with Mexico City and Guadalajara. It's also one of the fastest growing places of its size in North America, so it's rank and profile by population will only continue to go up.
San Antonio is in the center of three big dogs with Dallas to the north, Houston to the east, and Monterrey to the south. Downtown Houston is 197 miles from Downtown San Antonio, Downtown Dallas is 274 miles from Downtown San Antonio, and Centro Monterrey is 297 miles from Downtown San Antonio. SA's in the nexus and crossroads of these 3 big dogs.