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, Pentax X-5 has a
16.0MP 1/2.3-inch (6.08 x 4.56 mm ) sized BSI-CMOS sensor .
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 X-5 sensor size comparison.
As seen above, Pentax K-S2 has a 13.2x Larger sensor area than Pentax X-5. 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.