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 multi-electron 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 lower effective charge of the nucleus for them. The inner electrons' shielding effect diminishes in the order of .
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 the case of post-lanthenide elements like Hf, but it is not particularly efficient at protecting the outer shell. electrons with and . Zr is comparable to it.