Sigma DP1 has a 5.0MP APS-C (20.7 x 13.8 mm ) sized CMOS (Foveon X3) sensor . On the other hand, Ricoh GR III has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor .
Ricoh GR III's sensor provides 19MP more than Sigma DP1'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.
One other difference between these two cameras that is worth mentioning is that Ricoh GR III'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 moire occurring in certain scenes.
Below you can see the DP1 and GR III sensor size comparison.
As seen above, Ricoh GR III has a 1.3x Larger sensor area than Sigma DP1. 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.