Athens is about 2.3 times bigger than Porirua.

Porirua is approximately 182 sq km, while Athens (map shows its 34 named neighbourhoods; the area covers the whole city) is approximately 412 sq km, making Athens 126% larger than Porirua.
Tip: This to-scale comparison of Porirua vs. Athens uses the Mercator projection, which distorts the size of regions near the poles.
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