San Antonio is about 3.7 times bigger than Atlanta.

Atlanta (map shows its 114 named neighbourhoods; the area covers the whole city) is approximately 350 sq km, while San Antonio (map shows its 401 named neighbourhoods; the area covers the whole city) is approximately 1,292 sq km, making San Antonio 269% larger than Atlanta.
Tip: This to-scale comparison of Atlanta vs. San Antonio uses the Mercator projection, which distorts the size of regions near the poles.
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