Atlanta is about 4.1 times smaller than Saint Petersburg.

Saint Petersburg is approximately 1,439 sq km, while Atlanta (map shows its 114 named neighbourhoods; the area covers the whole city) is approximately 350 sq km, making Atlanta 24.36% the size of Saint Petersburg.
Tip: This to-scale comparison of Saint Petersburg vs. Atlanta uses the Mercator projection, which distorts the size of regions near the poles.
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