Canon SX20 IS has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized CCD sensor and features Digic 4 processor. On the other hand, Sony A550 has a
14.0MP APS-C (23.4 x 15.6 mm ) sized CMOS sensor and features Bionz processor.
Below you can see the SX20 IS and A550 sensor size comparison.
As seen above, Sony A550 has a 13.0x Larger sensor area than Canon SX20 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.