During mid-19th century there wasn't that many people there in Zacatecas, or at least north of it.....
Just under a million in 1850, well over if you include Zacatecas - easily enough to start a Mexican-American population (especially since their presence will bring others over the border), especially if the US is recieving less immigrants to water them down with. The really empty states you're thinking of at the time are Baja and Coahuila, everything else has got at least a hundred thousand or so Mexicans by mid-century - efforts to fill up the north were started right after the Texan revolution (its just a pity they didn't start with California and work their way south