Atlanta is about 2.3 times smaller than Kansas City.

Kansas City (map shows its 145 named neighbourhoods; the area covers the whole city) is approximately 815 sq km, while Atlanta (map shows its 114 named neighbourhoods; the area covers the whole city) is approximately 350 sq km, making Atlanta 43.03% the size of Kansas City.
Tip: This to-scale comparison of Kansas City vs. Atlanta uses the Mercator projection, which distorts the size of regions near the poles.
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