When Mexcio owned what is now the Southwest U.S., Texas and California, there were fewer than 75,000 Mexicans (some put the number as low as 50,000) in the entire area.
Virtually all of that number was in the lower Rio Grande valley. In California, the number never exceeded 7000 men, women and children of Spaniard or Mestizo descent.
The claim of "ownership" of the Southwest by Mexico was a farce at the time. Currently, they have a much stronger claim, which they will be undoubtedly asserting much more strenuously.
That’s because the comanches were tough customers to deal with. The people in New Mexico and Texas lived in fear of them for many years and even the soldiers of Mexico hated to make the trip up into New Mexico territory.