Both Pentax K-5 IIs and Nikon P510 have 16.0 MP resolution sensors but Pentax K-5 IIs'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-5 IIs'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.
Another difference between these two cameras is that Pentax K-5 IIs'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 K-5 IIs and P510 sensor size comparison.
As seen above, Pentax K-5 IIs 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.