why atomic radii of iron(Fe) and cobalt(Co) is equal

atomic radii of iron(Fe) and cobalt(Co) is equal:-
despite there is an increase number of protons which results in an increased nuclear attraction, the addition of an electron in a 3d subshell shields the 4s electrons better than when an electron is added to a subshell which is found in shell with the quantum number 4. This happens because the 3d electrons are in an inner shell.
Therefore, the effect of increasing nuclear charge will somewhat be neutralised by the increase in shielding effect due to the addition of an electron to the 3d subshell. This results in the effective nuclear charge to remain relatively constant as we move across the period. Therefore, the size of atoms of Fe and Co to be approximately the same.