Both Olympus TG-870 and Fujifilm X-A10 have 16.0 MP resolution sensors but Olympus TG-870's sensor is (1/2. ) and Fujifilm X-A10's sensor is APS-C (23.6 x 15.6 mm ).
Since Fujifilm X-A10'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.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-A10'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 moiré occurring in certain scenes.
Below you can see the TG-870 and X-A10 sensor size comparison.
As seen above, Fujifilm X-A10 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.