Solubility of Alkali Metal Chlorides
When dissolving, the ionic crystal’s lattice enthalpy must be broken while at the same time the hydration shell is generated around the cations and anions. The latter process is typically exergonic while the former is strongly endergonic.

How well a salt dissolves depends not only on the strength of solvation but also on the strength of the crystal lattice. If the lattice is strong, the solubility will be lower because there is an increased tendency to reform the crystal lattice.