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, Sigma fp L has a 61.0MP Full frame (36 x 24 mm ) sized BSI-CMOS sensor .
Sigma fp L's sensor provides 49MP 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 E-PL1s and fp L sensor size comparison.
As seen above, Sigma fp L has a 3.8x 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.