South Dakota is about 10 times smaller than Mexico.

Mexico is approximately 1,964,375 sq km, while South Dakota is approximately 196,540 sq km, making South Dakota 10.01% the size of Mexico. Meanwhile, the population of Mexico is ~129.2 million people (128.3 million fewer people live in South Dakota).
Tip: This to-scale comparison of Mexico vs. South Dakota uses the Mercator projection, which distorts the size of regions near the poles.