Lattice energy U is proportional to Z+ Z- and disproportional to the R0 ;
Z+ = charge of cation
Z- = charge of anion
R0 = R++R- where R+ = radious of cation and R- is radious of anion.
so higher the charge and lower the radious, higher will be lattice energy.
here anion is common that is oxide anion but the difference is cation one is Al3+ and other is Pb2+ .
as Al3+ has higher charge and lower radious than Pb2+ so Al3+ contribute more lattice energy and hence Al2O3 has more latice energy than PbO.