Yes, it has to change as the ions are smaller than atoms, but again, it will change at both ends so it is m1
I missed the example, it should be the cathode. So I imagine that there is only a cathode and at the cathode, the following process happens: Hg2+
Hg as Hg is bigger than Hg2+
the volume increases, because the solution volume decreases, but the solution won't move. 2 processes cause the move.