British Indian Ocean Territory is about 21,117 times smaller than Niger.

Niger is approximately 1,267,000 sq km, while British Indian Ocean Territory is approximately 60 sq km, making British Indian Ocean Territory 0.0% the size of Niger. Meanwhile, the population of Niger is ~27.9 million people (27.9 million fewer people live in British Indian Ocean Territory).
Tip: This to-scale comparison of Niger vs. British Indian Ocean Territory uses the Mercator projection, which distorts the size of regions near the poles.
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