Panasonic GX850 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Panasonic FX580 has a
12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor .
Another difference between these two cameras is that Panasonic GX850's sensor lacks 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 GX850 and FX580 sensor size comparison.
As seen above, Panasonic GX850 has a 8.1x Larger sensor area than Panasonic FX580. 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.