Samsung NX300 has a 20.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features DRIMe IV processor. On the other hand, Pentax K-3 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Prime III processor.
Another difference between these two cameras is that Pentax K-3'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 NX300 and K-3 sensor size comparison.
Samsung NX300 and Pentax K-3 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.