Both Ricoh WG-4 and Fujifilm XP120 have 16.0 MP resolution sensors but Ricoh WG-4's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Fujifilm XP120's sensor is (1/2. ).
Since Ricoh WG-4'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 Fujifilm XP120.
Below you can see the WG-4 and XP120 sensor size comparison.
As seen above, Ricoh WG-4 has a infx Larger sensor area than Fujifilm XP120. 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.