Both Samsung NX3000 and Samsung NX300 have 20.0 MP resolution sensors but Samsung NX3000's sensor is APS-C (23.5 x 15.7 mm ) and Samsung NX300's sensor is APS-C (23.5 x 15.7 mm ).
Since Samsung NX300's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Samsung NX3000.
Below you can see the NX3000 and NX300 sensor size comparison.
Samsung NX3000 and Samsung NX300 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.