Athens is about 2.5 times bigger than Stockton.

Stockton is approximately 164 sq km, while Athens (map shows its 34 named neighbourhoods; the area covers the whole city) is approximately 412 sq km, making Athens 151% larger than Stockton.
Tip: This to-scale comparison of Stockton vs. Athens uses the Mercator projection, which distorts the size of regions near the poles.
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