Fujifilm X-E2S has a 16.0MP APS-C (23.6 x 15.6 mm ) sized CMOS X-TRANS II sensor and features EXR Processor II processor. On the other hand, Panasonic GX8 has a 20.0MP Four Thirds (17.3 x 13 mm ) sized CMOS sensor and features Venus Engine processor.
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 GX8 sensor size comparison.
As seen above, Fujifilm X-E2S has a 1.6x Larger sensor area than Panasonic GX8. 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.