Olympus E-PL1s has a
12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Truepic V processor. On the other hand, Pentax K-5 IIs has a
16.0MP APS-C (23.7 x 15.7 mm ) sized CMOS sensor and features Prime II processor.
One other difference between these two cameras that is worth mentioning is that Pentax K-5 IIs'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-PL1s and K-5 IIs sensor size comparison.
As seen above, Pentax K-5 IIs has a 1.7x Larger sensor area than Olympus E-PL1s. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.