Olympus E-PM2 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor . On the other hand, Olympus E-PM1 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VI processor.
Below you can see the E-PM2 and E-PM1 sensor size comparison.
Olympus E-PM2 and Olympus E-PM1 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Olympus E-PM1 has 30% larger pixel area (18.45µm2 vs 14.12µm2) compared to Olympus E-PM2, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.