Sony WX220 has a 18.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Olympus E-PL1s has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Truepic V processor. Sony WX220's sensor provides 6MP more than Olympus E-PL1s's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the WX220 and E-PL1s sensor size comparison.
As seen above, Olympus E-PL1s has a 8.0x Larger sensor area than Sony WX220. 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.