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, Sony A57 has a 16.0MP APS-C (23.5 x 15.6 mm ) sized 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 moire occurring in certain scenes.
Below you can see the K-S2 and A57 sensor size comparison.
Pentax K-S2 and Sony A57 have the same 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 Sony A57 has 24% larger pixel area (22.87µm2 vs 18.37µm2) compared to Pentax K-S2, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.