energy required to remove an electron from an atom is defined as ionization enthalpy. Radius of cesium atom is 300 pm which is largest atom in periodic table. read more
And Cesium's ionic form, even with an empty outermost orbital, is less stable on account of more energy than elemental form. The only way you can get Cesium to ionise spontaneously (still with positive ionisation enthalpy) is to put a non-metal with it. read more
This also changes the shielding (from nuclear charge) of electrons in outer orbitals, and in the end, all $s$ orbitals are contracted and lowered in energy. $s$ orbitals are particularly affected because they have no nodal plane at the nucleus. read more