Canon 700D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 5 processor. On the other hand, Olympus E-620 has a 12.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic III+ processor.
Canon 700D's sensor provides 6MP more than Olympus E-620'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 700D and E-620 sensor size comparison.
As seen above, Canon 700D has a 1.5x Larger sensor area than Olympus E-620. 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.
DxOMark Sensor Scores
Both Canon 700D and Olympus E-620 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, 700D has scored 61, 6 points higher than E-620.