Both Fujifilm X-E2S and Fujifilm S8400W have 16.0 MP resolution sensors but Fujifilm X-E2S's sensor is APS-C (23.6 x 15.6 mm ) and Fujifilm S8400W's sensor is 1/2.3-inch (6.17 x 4.55 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 Fujifilm S8400W.
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 moiré occurring in certain scenes.
Below you can see the X-E2S and S8400W sensor size comparison.
As seen above, Fujifilm X-E2S has a 13.1x Larger sensor area than Fujifilm S8400W. 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.