San Francisco is about 3.4 times smaller than Athens.

Athens (map shows its 34 named neighbourhoods; the area covers the whole city) is approximately 412 sq km, while San Francisco (including the city proper only) is approximately 121 sq km, making San Francisco 29.37% the size of Athens.
Tip: This to-scale comparison of Athens vs. San Francisco uses the Mercator projection, which distorts the size of regions near the poles.
Our free app, MapSize by MyLifeElsewhere — compare any two places, to scale on your mobile device.