Lithium as well as caesium are both Group-1 elements.
Here, both Li^+ as well as Cs^+ have equal charges. But, the ionic size of the later is much more than that of the former.
Hence, the charge to size ratio of Li^+ is higher than that of Cs^+.
Now, higher the charge to size ratio of a cation, higher will be it's hydration enthalpy.
So, Li^+ will get immensely hydrated than that of negligible hydration of Cs^+.
Therefore, the hydrated Li^+ ion will be of much larger size than that of negligible hydrated Cs^+ ion in solution.
Hence, the mobility of Cs^+ ion will be much higher than that of Li^+ ion.
This means, that the Cs^+ ion will have higher conductivity than that of Li^+ ion in solution.
Hope it Helps:)