Fremont, California is 2.5 times larger than Concord, California.
Area: 80 km2 calculated
Concord is the largest city in Contra Costa County, California, in the United States. At the 2010 census, the city had a population of 122,067. Founded in 1869 as the community of Todos Santos by Salvio Pacheco, the name was changed to Concord within months. Wikipedia
Area: 200 km2 calculated
Fremont is a city in Alameda County, California. It was incorporated on January 23, 1956, from the merger of five smaller communities: Centerville, Niles, Irvington, Mission San Jose, and Warm Springs. Wikipedia