The atomic radii of Zr and Hf are similar. This is because:

of Lanthanoid contraction

Lanthanoid contraction is responsible for similar radii (e.g., Zr 160 pm, Hf 159 pm).

Zr and Hf have very similar physical and chemical properties much more than that expected on the basis of usual family relationship.

The factor responsible for the lanthanoid contraction is the imperfect shielding of one electron by another in the same set of orbitals. However, the shielding of one 4f electron by another is less than that of one d electron by another, and as the nuclear charge increases along the series, there is fairly regular decrease in the size of the entire 4fⁿ orbitals.