Tuvalu is about 31 times smaller than Kiribati.

Kiribati is approximately 811 sq km, while Tuvalu is approximately 26 sq km, making Tuvalu 3.21% the size of Kiribati. Meanwhile, the population of Kiribati is ~114,189 people (102,645 fewer people live in Tuvalu).
Tip: This to-scale comparison of Kiribati vs. Tuvalu uses the Mercator projection, which distorts the size of regions near the poles.