Enthalpy of reaction will be based off the relative stabilities of the products formed to the reactants.
CsOH is not very stable, as its radius is far larger than that of a hydroxide ion. Approximately sized ions provide the stablest compounds, as a rule of thumb.
LiOH, has two fairly matched ion sizes. Hence, the stability is far higher than that of CsOH, hence the enthalpy change is far higher, hence it has a higher heat of formation.
However, this is not the same as reactivity. Cs reacts violently with water in an explosion, as its 6s electron is weakly bound and heavily shielded from the nuclear. It is far easier to lose than the 2s electron on Li. So, this is why the reaction is far more violent for Cs than Li.
Is that okay?