Both Fujifilm XP90 and Olympus TG-870 have 16.0 MP resolution sensors but Fujifilm XP90's sensor is 1/2.3 (6.17 x 4.55 mm ) and Olympus TG-870's sensor is (1/2. ).
Since Fujifilm XP90'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 Olympus TG-870.
Below you can see the XP90 and TG-870 sensor size comparison.
As seen above, Fujifilm XP90 has a infx Larger sensor area than Olympus TG-870. 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.