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As $\ce{CH3COONa}$ is bulky, I thought that $\ce{CH3COONa}$ is a weak nucleophile. But I found out that it's a nucleophile good enough to perform $\mathrm{S_N2}$ reaction on a secondary carbon.

Why is it so?

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The acetate anion is actually not particularly bulky. The negative charge is located on the two oxygens which are approximately at a $120^\circ$ angle away from the only residue that has the potential to be bulky. Whatever that residues does will be ‘behind’ the carbon and hardly noticeable. However, they are still rather bad nucleophiles since the negative charge is delocalised over two oxygen atoms in the π system.

I’m actually not sure which orbital attacks nucleophilically but it could equally be an oxygen lone pair rather than the π system. If that is the case, there is really very little steric hindrance that could inhibit the reaction from proceeding. Thus, acetate is able to attack nucleophilically, as it does for example in a Mitsunobu reaction.

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