Chemistry: Molecular Approach (4th Edition)

The lattice energy of $BaO$ is much higher because the charge on its ions is higher.
An important factor in the magnitude of lattice energy is the product of the charges on the ions of the compound. In $BaO$, the cation and anion have a charge of 2 each, while in $CsF$, the cation and anion have a charge of 1 each. The product of the charges in BaO is 4 times of that in CsF. So, the lattice energy of $BaO$ is nearly 4 times that of $CsF$.