Olympus E-PL9 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VIII processor. On the other hand, YI M1 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor .
Below you can see the E-PL9 and M1 sensor size comparison.
Olympus E-PL9 and YI M1 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-PL9 has 26% larger pixel area compared to YI M1, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.