Sony A9 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized BSI-CMOS sensor and features BIONZ X processor. On the other hand, Samsung NX mini has a 20.5MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor .
Another difference between these two cameras is that Sony A9'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 A9 and NX mini sensor size comparison.
As seen above, Sony A9 has a 7.3x Larger sensor area than Samsung NX mini. 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.