Sony A55 has a
16.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Bionz processor. On the other hand, Panasonic G9 has a
20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
One other difference between these two cameras that is worth mentioning is that Panasonic G9's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the A55 and G9 sensor size comparison.
As seen above, Sony A55 has a 1.6x Larger sensor area than Panasonic G9. 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.