Brunei is about 43 times bigger than Christmas Island.

Christmas Island is approximately 135 sq km, while Brunei is approximately 5,765 sq km, making Brunei 4,170% larger than Christmas Island. Meanwhile, the population of Christmas Island is ~2,205 people (475,849 more people live in Brunei).
Tip: This to-scale comparison of Christmas Island vs. Brunei uses the Mercator projection, which distorts the size of regions near the poles.