Sigma fp has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor . On the other hand, Panasonic S1R has a 47.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Venus Engine processor.
Panasonic S1R's sensor provides 22MP more than Sigma fp'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 Panasonic S1R'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 fp and S1R sensor size comparison.
Sensor Size and Resolution Comparison image of Sigma fp and Panasonic S1R Cameras
Sigma fp and Panasonic S1R have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.