Nikon Z7 has a
46.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor and features Expeed 6 processor. On the other hand, Sigma SD1 Merrill has a
15.0MP APS-C (24 x 16 mm ) sized CMOS (Foveon X3) sensor and features Dual True II processor.
Nikon Z7's sensor provides 31MP 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 similarity between these two cameras is that both Sigma SD1 Merrill and Nikon Z7 sensors lack anti-alias (Low-Pass) filters. 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 Z7 and SD1 Merrill sensor size comparison.
As seen above, Nikon Z7 has a 2.2x 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.