Reducing property of alkali metals
Reducing property in aqueous phase depends upon (1)  enthalpy of sublimation (2)enthalpy of ionisation (3)enthalpy of hydration. 
First two processes are endothermic while last one is exothermic. 
For lithium enthalpy of hydration is very high because of its small size hence Li acts as stronge reducing agent in aqueous phase. For Cs enthalpy of hydration is very small hence overall process is less exothermic .