Cherish No. 6 while you can, Philadelphia, because the Lone Star State is coming for your spot.

San Antonio is the second-fastest-growing large city in the United States, according to new data released Thursday by the U.S. Census Bureau, and it will likely pass Philly as the sixth-most-populous city by 2021.

The Census Bureau says Philadelphia had an estimated population of 1,584,138 residents as of July 1, 2018, an increase of just under 4,000 residents from the same date in 2017. San Antonio, meanwhile, added more than 20,000 residents during the same time span, second only to fast-growing Phoenix, the most recent city to climb ahead Philadelphia in population in the Census Bureau's estimates.

Since 2014, San Antonio has added roughly 24,690 residents per year, while Philadelphia has added just over 5,150 residents per year. At those paces, San Antonio will make up the remaining 51,905-resident gap between the two cities by the time the Census Bureau releases its 2021 population estimates.

The growth rates shouldn't be a huge surprise to Philadelphians. The West and Southwest, where land is abundant, have significantly outpaced population growth in the rest of the United States for the past half-decade:

The northeast, already crowded and dense, simply doesn't have comparable space at its disposal. San Antonio's land area is 460.93 square miles, according to the Census Bureau, while Philadelphia's land area is just 134.10 square miles.

When San Antonio moves past Philadelphia in the Census Bureau's estimates, Texas will own 33 percent of the top six cities in the country. Houston currently sits at No. 4 with an estimated 2.32 million residents, and is closing fast on Chicago, which has lost more than 22,000 residents since 2014, according to the Census Bureau.

New York City, of course, remains the most populated city in the United States by an enormous margin with an estimated 8,398,748 residents, though the city has actually seen it population decrease each year since 2015, according to estimates.

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