Canon S100 has a
12.0MP 1/1.7-inch (7.44 x 5.58 mm ) sized CMOS sensor and features Digic 5 processor. On the other hand, Canon A3000 IS has a
10.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor .
Below you can see the S100 and A3000 IS sensor size comparison.
As seen above, Canon S100 has a 1.5x Larger sensor area than Canon A3000 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.