Pentax K-S2 has a
20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor. On the other hand, Canon 100D has a
18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 5 processor.
Another difference between these two cameras is that Pentax K-S2'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 moiré occurring in certain scenes.
Below you can see the K-S2 and 100D sensor size comparison.
As seen above, Pentax K-S2 has a 1.1x Larger sensor area than Canon 100D. 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.