Both Pentax K-500 and Nikon P510 have 16.0 MP resolution sensors but Pentax K-500's sensor is APS-C (23.7 x 15.7 mm ) and Nikon P510's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Pentax K-500's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon P510.
Below you can see the K-500 and P510 sensor size comparison.
As seen above, Pentax K-500 has a 13.3x 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.