British Indian Ocean Territory is about 20,670 times smaller than Mali.

Mali is approximately 1,240,192 sq km, while British Indian Ocean Territory is approximately 60 sq km, making British Indian Ocean Territory 0.0% the size of Mali. Meanwhile, the population of Mali is ~25.2 million people (25.2 million fewer people live in British Indian Ocean Territory).
Tip: This to-scale comparison of Mali vs. British Indian Ocean Territory uses the Mercator projection, which distorts the size of regions near the poles.
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