Here's why:

* Molecular size: Nitrogen molecules (N₂) consist of two nitrogen atoms bonded together, while oxygen molecules (O₂) consist of two oxygen atoms bonded together.

* Atomic radii: Nitrogen atoms have a slightly larger atomic radius than oxygen atoms. This means that the nitrogen atoms in a nitrogen molecule are slightly farther apart than the oxygen atoms in an oxygen molecule.

* Bond length: The bond length between the two nitrogen atoms in N₂ is also slightly longer than the bond length between the two oxygen atoms in O₂.

Therefore, nitrogen molecules are slightly larger than oxygen molecules in terms of both atomic radius and bond length.