When considering the electrical conductivity of metal oxides, they will conduct better if the metals orbitals can reach each other to overlap. Whether this can happen is contributed to by a couple of things such as Z-eff charge (electronegativity), the anion, oxidation state etc. We're told that one of those properties is that as you down a group, the size of the orbitals increases, and hence more overlap.
But isn't making the atoms bigger just going to mean that the distances between the atoms get bigger proportionally? So switching a 3d for a 4d, but keeping the same structure would just look like "zooming in", but not change any proportional overlap?
As I understand f orbitals are inside (having an electron density maximum closer to the nucleus), and so don't participate in bonding.