California, Utah, Nevada, Arizona, Texas, New Mexico, and parts of Colorado, Oklahoma, Kansas, and even as far north was Wyoming, were all once part of Mexico, but the US got those states in the Mexican cession. It's kind of hard to imagine parts of the Great Plains, the Rockies, and even the Pacific Northwest (northern Cali) once being a part of mexico, but what if it stayed that way?
If those states were still part of Mexico, how would history have changed significantly? On a superficial level, California wouldnt be the popular tourist attraction it is now (or maybe it would be) since Los Angeles and San Fransisco would be in another country, but there are other things that would have changed.
One of them would be culture. There were a few native american southwest tribes that also lived in Mexico (who many Mexicans are descendants of today), but because of political reasons among others (Mexico has separate borders from the US), Mexicans arent identified with native Americans. Now, with a good chunk of what is now the US being a part of Mexico, native american tribes like the Pueblo, Navajo, wouldnt be considered native Americans. Even many Plains Indians and Pacific Northwest tribes would be considered Mexicans today
Other than culture, I cant really think of how having less states would have a significant effect on US history, other than possible changes in political election votes due to a smaller US population.