Both Nikon L31 and Pentax X-5 have 16.0 MP resolution sensors but Nikon L31's sensor is 1/2.3' (6.17 x 4.55 mm ) and Pentax X-5's sensor is 1/2.3-inch (6.08 x 4.56 mm ).
Since Nikon L31'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 L31 and X-5 sensor size comparison.
Sensor Size and Resolution Comparison image of Nikon L31 and Pentax X-5 Cameras
Nikon L31 and Pentax X-5 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.