Olympus E-PM1 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VI processor. On the other hand, Olympus E-PM2 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Below you can see the E-PM1 and E-PM2 sensor size comparison.
Olympus E-PM1 and Olympus E-PM2 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 compared to Olympus E-PM2, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.