Sigma Quattro H has a 45.0MP APS-H (26.6 x 17.9 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor. On the other hand, Olympus PEN-F has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features TruePic VII processor.
Sigma Quattro H's sensor provides 25MP more than Olympus PEN-F'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.
Another difference between these two cameras is that Sigma Quattro H's sensor lacks 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 moire occurring in certain scenes.
Below you can see the Quattro H and PEN-F sensor size comparison.
As seen above, Sigma Quattro H has a 2.1x Larger sensor area than Olympus PEN-F. 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.