Two open beakers, one beaker containing a solvent and the other beaker containing a mixture of that solvent with a non volatile solute are together sealed in a container. Over time:

As solvent is pure in beaker (I) and volatile, so it will vaporize and increase vapor pressure till that vapor pressure will come in equilibrium so volume decrease.

In second beaker (II), there is a solution having a non-volatile solute and solvent. As particles are condensing from the vapor phase in beaker II, the vapor pressure will decrease.

Therefore, the volume of the solution increases and the volume of the solvent decreases.