In ${Na}(186 pm)$ and ${K}$ (231 pm), and ${Mg}$ $(160 {pm})$ and ${Ca}(180 {pm})$ there is adequate increase in atomic size which is not observed in case of ${Al}(118 {pm})$ and ${Ga}(135 {pm})$.
As size of the atom increases, ionisation energy decreases thus, there is no substantial decrease in ionisation enthalpy in case of ${Al}$ and ${Ga}$.