Leica S2 has a 38.0MP Medium format (45 x 30 mm ) sized CCD sensor . On the other hand, Nikon D6 has a 21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor.
Leica S2's sensor provides 17MP more than Nikon D6'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 S2 and D6 sensor size comparison.
![Leica S2 vs Nikon D6 Sensor Size Comparison]()
Sensor Size and Resolution Comparison image of Leica S2 and Nikon D6 Cameras
As seen above, Leica S2 has a 1.6x Larger sensor area than Nikon D6. 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.