The seeds swell when placed in water because of imbibition. Imbibition is the process when solid surfaces absorb water and swells.
It is the phenomena wherein water adsorption takes place by substances without solution formation. When seeds are immersed in water, they swell due to imbibition. This swelling up causes a temporary increase in the volume of the cell and does not require energy as materials are transported passively.
Raisins increase in size due to imbibition of water. The quantity of water imbibed can be obtained by considering the difference in the mass between the dry raisins and the swollen raisins.