Explain why is there a phenomenal decrease in ionisation enthalpy from carbon to silicon?

As we move from carbon to silicon atomic size increases, i.e., the distance between the outermost electron and nucleus increases. Thus, this electron experience very small attraction from the nucleus and hence, easy to remove. Since, the size of Si atom is larger, the outer electron experience lesser attraction and hence, its ionisation enthalpy (energy required to remove an electron) is smaller.