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:)
