Pentax K-3 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Prime III processor. On the other hand, Pentax K-S2 has a 20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor.
Another similarity between these two cameras is that both Pentax K-S2 and Pentax K-3 sensors lack anti-alias (Low-Pass) filters. 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 K-3 and K-S2 sensor size comparison.
Pentax K-3 and Pentax K-S2 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Pentax K-S2 has 20% larger pixel area (18.37µm2 vs 15.24µm2) compared to Pentax K-3, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.