We investigated the swelling effect in the thermotropic smectic C liquid crystal phase by dilution of the solvent. We found that the twist elastic constant monotonically decreases with increasing swelling ratio, eventually becoming an order of magnitude smaller than its original value at the swelling limit. Our conclusion is that intercalated solvent molecules prevent direct collisions between liquid crystal molecules, which weakens the steric interaction to produce the orientation order across the smectic layers. This is the main mechanism for the softening of the twist elastic constant upon swelling.