Is the -OCH3 group bulky?
In organic chemistry bulkiness isn't just a matter of geometry; one can find the volume of the group using vdW radii, but it's a crude approximation.
Quantitatively steric bulk relies on energy measurements and can be described with a cyclohexane A-value — energy required for a group to switch between axial and equatorial positions in cyclohexane.
From the table of A-values methoxy group is indeed less bulk in comparison to nitro and methyl groups:

$$
\begin{array}{lc}
\hline
\text{Substituent} &amp; \text{A-Value}/\pu{kcal mol-1} \\
\hline
\ce{-OCH3} &amp; 0.6 \\
\ce{-NO2} &amp; 1.1  \\
\ce{-CH3} &amp; 1.7 \\
\hline
\end{array}
$$