Canon SX120 IS has a 10.0MP 1/2.5-inch (5.744 x 4.308 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Canon D30 has a 12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features DIGIC 4 processor.
Below you can see the SX120 IS and D30 sensor size comparison.
As seen above, Canon D30 has a 1.1x Larger sensor area than Canon SX120 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.