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, Olympus E-PL1 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Truepic V processor.
Below you can see the E-PL9 and E-PL1 sensor size comparison.
Olympus E-PL9 and Olympus E-PL1 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. On the other hand, since Olympus E-PL1 has 30% larger pixel area (18.45µm2 vs 14.12µm2) compared to Olympus E-PL9, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.