Both Olympus E-PM2 and Fujifilm F750EXR have 16.0 MP resolution sensors but Olympus E-PM2's sensor is Four Thirds (17.3 x 13 mm ) and Fujifilm F750EXR's sensor is 1/2-inch (6.4 x 4.8 mm ).
Since Olympus E-PM2'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 F750EXR.
Below you can see the E-PM2 and F750EXR sensor size comparison.
As seen above, Olympus E-PM2 has a 7.3x Larger sensor area than Fujifilm F750EXR. 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.