Fujifilm X-A3 has a 24.0MP APS-C (23.5 x 15.7 mm ) sized CMOS sensor and features EXR Processor II processor. On the other hand, Samsung NX500 has a 28.0MP APS-C (23.5 x 15.7 mm ) sized BSI-CMOS sensor and features DRIMe 5 processor.
One other difference between these two cameras that is worth mentioning is that Samsung NX500's sensor doesn't have an 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 X-A3 and NX500 sensor size comparison.
Fujifilm X-A3 and Samsung NX500 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Fujifilm X-A3 has 16% larger pixel area compared to Samsung NX500, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.