Nikon D6 has a
21.0MP Full frame (35.9 x 23.9 mm ) sized CMOS sensor and features Expeed 6 processor. On the other hand, Nikon S7000 has a
16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor .
Nikon D6's sensor provides 5MP more than Nikon S7000'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 D6 and S7000 sensor size comparison.
As seen above, Nikon D6 has a 30.6x Larger sensor area than Nikon S7000. 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.