Borek is correctit is a hydration effect.
[M(H2O)n]^+ n includes primary hydration sphere (directly bound to M^+) and secondary hydration sphere
The smaller the ion the greater the electrostatic field, the greater n, the slower the ionic mobility:
Li^+ n= ~25.3, Ω = 33.5; Na^+ n= ~16.6, Ω = 43.5; K^+ n= ~10.5, Ω = 64.6; Cs^+ n= ~9.9 Ω = 68.
F. A. Cotton, G. Wilkinson, C. A. Murillo, M. Bochmann, Advanced Inorganic Chemistry 6th ed (1999). p 102