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

Ionisation enthalpy of carbon (the first element of group 14) is very high ( $1086 \mathrm{~kJ} / \mathrm{mol}$ ). This is expected owing to its small size. However, on moving down the group to silicon, there is a sharp decrease in the enthalpy (786 kJ). This is because of an appreciable increase in the atomic sizes of elements on moving down the group.