Ricoh G900 has a 20.0MP 1/2.3 (6.17 x 4.55 mm ) sized BSI-CMOS sensor . On the other hand, Panasonic FX580 has a 12.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized CCD sensor .
Ricoh G900's sensor provides 8MP more than Panasonic FX580's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the G900 and FX580 sensor size comparison.
Ricoh G900 and Panasonic FX580 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.