Nikon D750 has a 24.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Expeed 4 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 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 D750 and K-S2 sensor size comparison.
As seen above, Nikon D750 has a 2.4x Larger sensor area than Pentax K-S2. 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.