Both Nikon D610 and Pentax KP have 24.0 MP resolution sensors but Nikon D610's sensor is Full frame (35.9 x 24 mm ) and Pentax KP's sensor is APS-C (23.5 x 15.6 mm ).
Since Nikon D610'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 Pentax KP.
Below you can see the D610 and KP sensor size comparison.
As seen above, Nikon D610 has a 2.4x Larger sensor area than Pentax KP. 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.