Expression comparing the Solubility of Ionic compounds

In chemical bonding, I am taught an expression that compares the Solvation Energy and the Lattice Energy of an ionic compound which is being dissolved in a polar solvent (could be water, could be something else).  

The expression is:  

$$\left\lvert r\_+ -r\_-\right\rvert$$
where $r\_+$ denotes radius of cation and $r\_-$ denotes radius of anion making up the compound. When this difference is large, solvation energy dominates and when this difference is small, lattice energy dominates. But I don't the reason behind these deductions. Can someone explain?


