Illinois is about 310 times bigger than Northern Mariana Islands.

Northern Mariana Islands is approximately 464 sq km, while Illinois is approximately 143,961 sq km, making Illinois 30,926% larger than Northern Mariana Islands. Meanwhile, the population of Northern Mariana Islands is ~51,475 people (12.8 million more people live in Illinois).
Tip: This to-scale comparison of Northern Mariana Islands vs. Illinois uses the Mercator projection, which distorts the size of regions near the poles.