The solubility of an ionic compound depends on two factors :
(i) lattice energy and
(ii) hydration energy.
These two factors oppose each other.
If lattice energy is high, the ions will be tightly packed in the crystal and therefore, solubility will be low. If the hydration energy is high, the ions will have greater tendency to be hydrated and therefore, solubility will be high.
Alkali metals i.e., sodium or potassium hydroxides will be highly soluble in water due to larger size of Na and K as compared to that of Mg and Ca, the lattice energies of hydroxides and carbonates of sodium and potassium are much lower than those of hydroxides and carbonates of magnesium and calcium. As a result, the hydroxides and carbonates of Na and K are easily soluble in water while the corresponding salts of Mg and Ca are sparingly soluble in water.