Both Nikon L31 and Fujifilm X-Pro1 have 16.0 MP resolution sensors but Nikon L31's sensor is 1/2.3' (6.17 x 4.55 mm ) and Fujifilm X-Pro1's sensor is APS-C (23.6 x 15.6 mm ).
Since Fujifilm X-Pro1'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 L31.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-Pro1's sensor doesn't have an 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 moire occurring in certain scenes.
Below you can see the L31 and X-Pro1 sensor size comparison.
As seen above, Fujifilm X-Pro1 has a 13.1x Larger sensor area than Nikon L31. 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.