Nikon S33 has a 13.0MP 1/3.1" (4.7 x 3.5 mm ) sized CMOS sensor . On the other hand, Nikon S31 has a 10.0MP 1/2.9" (4.96 x 3.72 mm ) sized CCD sensor .
Below you can see the S33 and S31 sensor size comparison.
As seen above, Nikon S31 has a 1.1x Larger sensor area than Nikon S33. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.