Ricoh G900 has a 20.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor . On the other hand, Sigma DP2 has a 5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor .
Ricoh G900's sensor provides 15MP more than Sigma DP2'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 DP2 sensor size comparison.
As seen above, Sigma DP2 has a 10.2x Larger sensor area than Ricoh G900. 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.