We didn’t seize their land, they sold it to us.
Actually, I think both are true. We beat them in a war for the territory, then turned around and paid them for the land.
Treaty of Guadelupe.
They did not sell most of the land to us. We acquired over 500,000 square miles of Mexican territory by aggressive warfare. Google “Treaty of Guadalupe Hidalgo” and read a few articles. Here is one I found interesting: https://www.mtholyoke.edu/~jasingle/conclus.html
Only the Gadsden Purchase was a negotiated sale. It was just under 30,000 square miles and served several useful purposes as described here: https://en.wikipedia.org/wiki/Gadsden_Purchase