From the Born–Landé equation the lattice energy U ∝ (Z+ × Z-) / (r+ + r-) where Z+ and Z- are the charges on the cation and anion, respectively; r+ and r- are radii of the cation and anion, respectively.
The Z+×Z- term dominates. MgO has 2×2 (4) so it will have the higher U than Li2O Z+ × Z- = 2.
You only consider (r+ + r-) term when the Z+×Z- term is the same; smallest the (r+ + r-), the larger is the U.
U(MgO) = 3795 kJ mol^-1
U(Li2O) = 2799 kJ mol^-1
MgO has larger.
