Nikon D3400 has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features Expeed 4 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 Nikon D3400'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 D3400 and NX mini sensor size comparison.
As seen above, Nikon D3400 has a 3.2x 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.