New York City is about 25 times smaller than New Jersey.

New Jersey is approximately 19,211 sq km, while New York City (including all five boroughs: Manhattan, Brooklyn, Queens, The Bronx, and Staten Island) is approximately 781 sq km, making New York City 4.07% the size of New Jersey.
Tip: This to-scale comparison of New Jersey vs. New York City uses the Mercator projection, which distorts the size of regions near the poles.