Sony A7R has a
36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Bionz X processor. On the other hand, Nikon P510 has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor.
Sony A7R's sensor provides 20MP more than Nikon P510'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 Sony A7R'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 A7R and P510 sensor size comparison.
As seen above, Sony A7R has a 30.7x Larger sensor area than Nikon P510. 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.