Both Fujifilm X-A2 and Olympus E-PM2 have 16.0 MP resolution sensors but Fujifilm X-A2's sensor is APS-C (23.6 x 15.6 mm ) and Olympus E-PM2's sensor is Four Thirds (17.3 x 13 mm ).
Since Fujifilm X-A2'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 E-PM2.
Below you can see the X-A2 and E-PM2 sensor size comparison.
As seen above, Fujifilm X-A2 has a 1.6x Larger sensor area than Olympus E-PM2. 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.