Imbibition is the adsorption of water molecules on to colloidal sized particles. In these seeds, the cellulose fibers of the cell walls are colloidal sized. The walls swell as water saturates the surfaces of these fibers. The difference between the size of the peas in 50% sucrose and 100 % water is related to the rate of diffusion of water into the pea. Given time, the peas in the aqueous media would all swell to an equal size. Petroleum ether is a non polar molecule and is not attracted to the charged surfaces of the cellulose fibers.
The phenomenon seen here is not related to osmosis.