The size for Zr and Hf is same because of lanthanide contraction.
The electrons are added in outer shells in multi-electron atoms. The electrons which is present in inner shells makes them experience a lower effective charge of the nucleus, shield the outer electrons from nuclear charge. The inner electrons shielding effect exerted decreases in the order s > p > d > f. This f-subshell poorly shields the outer electron from nuclear attraction which results in more attractive pull of nucleus on outer electron and smaller size. In case of post lanthenide elements like Hf, 4f subshell is filled and it is not very effective at shielding the outer shell (n=5 and n=6) electrons. It is similar to Zr.