Canon S90 has a
10.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon A1200 has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features DIGIC 4 with iSAPS technology processor.
Below you can see the S90 and A1200 sensor size comparison.
As seen above, Canon S90 has a 1.5x Larger sensor area than Canon A1200. 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.