Olympus E-P1 has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic V processor. 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 E-P1 and E-PL9 sensor size comparison.
Olympus E-P1 and Olympus E-PL9 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-P1 has 30% larger pixel area compared to Olympus E-PL9, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.