Sigma SD1 Merrill has a
15.0MP APS-C (24 x 16 mm ) sized CMOS (Foveon X3) sensor and features Dual True II processor. On the other hand, Fujifilm GFX 50R has a
51.0MP Medium format (44 x 33 mm ) sized CMOS sensor and features X Processor Pro processor.
Fujifilm GFX 50R's sensor provides 36MP more than Sigma SD1 Merrill'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.
Another difference between these two cameras is that Sigma SD1 Merrill'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 SD1 Merrill and GFX 50R sensor size comparison.
As seen above, Fujifilm GFX 50R has a 3.8x Larger sensor area than Sigma SD1 Merrill. 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.