London is about 4.4 times bigger than Detroit.

Detroit (map shows its 92 named neighbourhoods; the area covers the whole city) is approximately 359 sq km, while London (including the 32 boroughs of greater London) is approximately 1,572 sq km, making London 338% larger than Detroit.
Tip: This to-scale comparison of Detroit vs. London uses the Mercator projection, which distorts the size of regions near the poles.
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