Both Fujifilm X-E2S and Panasonic GX850 have 16.0 MP resolution sensors but Fujifilm X-E2S's sensor is APS-C (23.6 x 15.6 mm ) and Panasonic GX850's sensor is Four Thirds (17.3 x 13 mm ).
Since Fujifilm X-E2S'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 Panasonic GX850.
Another difference between these two cameras is that Fujifilm X-E2S'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 X-E2S and GX850 sensor size comparison.
As seen above, Fujifilm X-E2S has a 1.6x Larger sensor area than Panasonic GX850. 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.