The lanthanide contraction 1s responsible for the fact that:

The lanthanide contraction 1s responsible for the fact that is Zr and Hf have about the same radius.

The size of Zr and Hf is the same because of lanthanide synthesis. The shielding effect can be used to explain lanthanide contraction. The electrons in multielectron atoms are added to the outer shells. The outer electrons are protected from nuclear charge by the electrons already in the inner shells, resulting in a reduced effective charge of the nucleus for them. The inner electrons' shielding effect diminishes in the following order: s > p > d > f. This f subshell does a poor job of protecting the outer electron from nuclear attraction, which leads to a stronger pull from the nucleus and a smaller size. The 4f subshell is filled in post-lanthenide elements like Hf, however it is not particularly successful at securing the outer shell (n=5 and n=6) electrons.

Zr is comparable to it.