Lattice energy is also decreasing down the group. hence thermal stability is decreasing. But since lattice energy is decreasing squarely and hydration energy is decreasing linearly, the effect of decrease in lattice energy is higher than the effect of decrease in hydration energy. So if energy required to break lattice is very less, then the lattice of the bottom elements in alkali group will break easily compared to the lighter ones. Thus if not lattice form, then they prefer the aqueous state and hence they love to stay dissolved and not form lattice. So solubility increases.