The United States actually did annex a large portion of Mexico after the war -- not at all coincidentally, the portions it was trying to buy from Mexico before the war. The US also annexed, of course, the Texan republic; which was a main cause of the war.

The Texas annexation added this area to the U.S., the portion of which in green was claimed by Mexico. The Treaty of Guadalupe Hidalgo which ended the war added this area to the U.S., meaning the total area annexed included the modern U.S. states of California, Texas, Nevada, Utah, and also portions of modern Arizona, New Mexico, Colorado, Wyoming, Oklahoma and Kansas. If we include the later Gadsden Purchase in the area lost by Mexico to the U.S., Mexico lost about 55% of its area.