Carbonate salts have a very large anion so hydration energy should dominate over lattice energy. Since hydration energy is inversely proportional to radius of ion, I would expect Lithium to release the largest hydration energy and thus be the most soluble. But my textbook (Concise Inorganic Chemistry by JD LEE) says that the "Solubility of carbonates and fluorides of Alkali metals increase rapidly on descending the group. The reason for this is that their Lattice energies change more than the hydration energies on descending the group." However, it provides no further explanation as to why that is true. Could someone please explain this better?