It's really not. For comparison, Vermont (not a particularly populous New England state) had 315,000 people and fielded 33,000 soldiers. There's a reason California only had 2 representatives in the House.
true, but most of the Anglo population in California is in the Stockton/San Francisco/Sacramento triangle, while the future megalopolis of Los Angeles is a sleepy little town in 1861 with a Hispanic population only somewhat outnumbered by the Anglo one. The future cities of San Diego as well as those in the Central Valley are villages or small towns at this point. So really those 16,000 troops in OTL had little to do in California other than guard San Francisco and beat up Indians and some of that Indian fighting was in Arizona and New Mexico (they were important in numbers against the Navajo and Apache). As either of those campaigns is of far less importance than keeping California in the Union if it becomes an issue they stay home and hang would be Confederates