Question

Why does hydration bond strength depends on atomic size?

Answer 1

Hydration energy is a term for energy released upon attachment of water molecules to ions. The ions become covered by the neighboring water molecules.
Electrostatic attractions of ions for water (hydration) depend on the intensity of the charge and the distance between interacting centers. Smaller ions have charge more concentrated than larger ones. And water can get closer to smaller ions than to larger ions. Thus lithium has the highest hydration enthalpy in Group 1 of Periodic Table.