This atomic radius of Al is larger therefore the valence electron is further away from the nucleus. 

The valence electron is removed from the 3p orbital for Al rather than 3s orbital for Mg

The valence electron in Al is less shielded from the nucleus and so the electron will observe a larger effective nuclear charge. 

The valence electron is removed from the 3s orbital for Al rather than 3p orbital for Mg

The valence electron in Mg is more shielded from the nucleus and so the electron will observe a larger effective nuclear charge.

The electronegativity of Mg is greater than Al and so will result in Mg having a higher tendency to attract covalently bound electrons.