Canon SD780 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor . On the other hand, Canon S90 has a
10.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CCD sensor and features Digic 4 processor.
Below you can see the SD780 IS and S90 sensor size comparison.
As seen above, Canon S90 has a 1.5x Larger sensor area than Canon SD780 IS. 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.