Both Olympus E-PM2 and Olympus E-PL8 have 16.0 MP resolution sensors but Olympus E-PM2's sensor is Four Thirds (17.3 x 13 mm ) and Olympus E-PL8's sensor is Four Thirds (17.3 x 13 mm ).
Since Olympus E-PL8'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 E-PM2 and E-PL8 sensor size comparison.
Olympus E-PM2 and Olympus E-PL8 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.