The Mexican-American War was notable for a strong anti-war party, active before, during, and after the war. This was because they were anchored by the Abolitionists. Anti-war people saw no reason to interfere with a fellow republic, and especially not to kill people to support a war to benefit only Southern slave owners. Manifest Destiny was invoked, and trumpeted, and promoted to give Northeners some sense of benefit to it.

You see, without scientific agriculture, cotton wore out the land. It was perceived that cotton planters needed new lands to move to (buying new and selling old) that were below the Mason-Dixon Line and could become new slave territories and states. That was why so many moved to Texas and rebeled when their national gov't - Mexico - outlawed slavery. That was why many elements of the USA had been reluctant to admit the Republic of Texas to the Union.

So the Abolitionists, and the Free Soil Party, and others, were happy to minimize the amount of Mexico taken. They tried to keep it to areas with a strong white and European presence rather than taking on large swaths only inhabited by mostly native people. (The Free Soil Party wanted free white soil and one reason to keep out slavery was to keep out freedmen.) They were a bit stuck with the "tidy lines" drawn out from Texas, but the geography drew those farther north to the head of the gulf of California.

LSS: The same people who tried to prevent the war happening tried to minimize territorial gains for the South and slavery.