YI M1 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor . On the other hand, Olympus E-PL9 has a 16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VIII processor.
Below you can see the M1 and E-PL9 sensor size comparison.
YI M1 and Olympus E-PL9 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 (14.12µm2 vs 11.16µm2) compared to YI M1, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.