Sony A37 has a 16.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor . 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.
One other difference between these two cameras that is worth mentioning is that Pentax K-S2's sensor doesn't have an 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 A37 and K-S2 sensor size comparison.
Sony A37 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 Sony A37 has 24% larger pixel area compared to Pentax K-S2, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.