White House is about 13 times smaller than Colosseum.

Colosseum is approximately 0.019 km² (0.0075 sq mi), while White House is approximately 0.0015 km² (0.00058 sq mi), making White House 7.77% the size of Colosseum.
Tip: This to-scale comparison of Colosseum vs. White House uses the Mercator projection, which distorts the size of regions near the poles.
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