Olympus E-PL8 has a
16.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VII processor. On the other hand, Olympus E-P7 has a
20.0MP Four Thirds (17.4 x 13 mm ) sized CMOS sensor and features TruePic VIII processor.
One other difference between these two cameras that is worth mentioning is that Olympus E-P7's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the E-PL8 and E-P7 sensor size comparison.
Olympus E-PL8 and Olympus E-P7 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.