Nikon S31 has a
10.0MP 1/2.9-inch (4.96 x 3.72 mm ) sized CCD sensor . On the other hand, Canon SD780 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the S31 and SD780 IS sensor size comparison.
As seen above, Canon SD780 IS has a 1.5x Larger sensor area than Nikon S31. 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.