The correct order of the solubility of alkaline-earth metal sulphates in water is:

Hydration power energy $\propto$  ${\text{q}}_{\text{1}}{\text{q}}_{\text{2}}\left(\frac{\text{1}}{{\text{r}}_{+}}\text{+}\frac{\text{1}}{{\text{r}}_{-}}\right)$

$\therefore$ Solubility order of IIA group element sulphates in water:

$\mathrm{M}\mathrm{g}\mathrm{S}{\mathrm{O}}_4>\mathrm{C}\mathrm{a}\mathrm{S}{\mathrm{O}}_4>\mathrm{S}\mathrm{r}\mathrm{S}{\mathrm{O}}_4>\mathrm{B}\mathrm{a}\mathrm{S}{\mathrm{O}}_4$

Generally bigger size ion form a strong bond with bigger ion so solubility will decrease or smaller the cation, greater the hydration sphere. 
Hence, Solubility of ${\mathrm{SO}}_4^{-2}$ ion with alkali or alkaline earth metal decrease top to bottom.