Oklahoma City is 4.1 times larger than Montgomery, Alabama.
Area: 411 km2 calculated
Montgomery is the capital of the State of Alabama, and is the county seat of Montgomery County. Named for Richard Montgomery, it is located on the Alabama River, in the Gulf Coastal Plain. As of the 2010 Census, Montgomery had a population of 205,764. Wikipedia
Area: 1,682 km2 calculated
Oklahoma City is the capital and largest city of the U.S. state of Oklahoma. The county seat of Oklahoma County, the city ranks 27th among United States cities in population. The population grew following the 2010 Census, with the population estimated to have increased to 610,613 as of July 2013. Wikipedia