Both Pentax X-5 and Pentax K-5 II have 16.0 MP resolution sensors but Pentax X-5's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Pentax K-5 II's sensor is APS-C (23.7 x 15.7 mm ).
Since Pentax K-5 II'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 X-5.
Below you can see the X-5 and K-5 II sensor size comparison.
As seen above, Pentax K-5 II has a 13.4x Larger sensor area than Pentax X-5. 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.