Let's denote the population of Chicago as \( x \). According to the information given, the population of New York City is about 9.9 million more than the population of Chicago. This can be expressed with the following equation:
\[ 18.8 \text{ million} = x + 9.9 \text{ million} \]
To solve for \( x \), we rearrange the equation:
\[ x = 18.8 \text{ million} - 9.9 \text{ million} \]
Now, we perform the subtraction:
\[ x = 8.9 \text{ million} \]
Thus, the approximate population of Chicago is \( \textbf{8.9 million} \).