Jacksonville is about 5 times bigger than Detroit.

Detroit (map shows its 92 named neighbourhoods; the area covers the whole city) is approximately 359 sq km, while Jacksonville (map shows its 197 named neighbourhoods; the area covers the whole city) is approximately 1,935 sq km, making Jacksonville 439% larger than Detroit.
Tip: This to-scale comparison of Detroit vs. Jacksonville uses the Mercator projection, which distorts the size of regions near the poles.
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