Answer
The trend in lattice energy shown is not due to differences in ionic radii.
Work Step by Step
From Coulomb's law $$E_{el}=\frac{kQ_1Q_2}{d}$$ It has been shown that lattice energy depends on both the charges of the ions and the distance among the ions (which can be predicted from ionic radii).
However, the charges of the ions is the more profound determinant of lattice energy. Therefore, in cases where the compounds have different-numbered charged ions, to determine which has higher or lower lattice energy, we rely almost only on the charges of the ions of each compound.
But in cases where the compounds have similarly charged ions (for example, both singly or doubly charged ions), we have no other choice but to look at the distance among the ions, or look at ionic radii, to determine which compound has higher or lower lattice energy.
Therefore, only in cases where the compounds have similar-numbered charged can we shows trends in lattice energy due to differences in ionic radii.
$$BaO\gt KF$$
$BaO$ is made up of $Ba^{2+}$ and $O^{2-}$ ions, and $KF$ is made up of $K^+$ and $F^-$ ions.
Since 1 compound is made up of doubly-charged ions and 1 compound is made up of singly-charged ions, the trend in lattice energy shown is not due to differences in ionic radii.
