Fujifilm X-H2 has a
40.0MP APS-C (23.5 x 15.6 mm ) sized BSI-CMOS sensor and features X-Processor 5 processor. On the other hand, Leica S2 has a
38.0MP Medium format (45 x 30 mm ) sized CCD sensor .
Another difference between these two cameras is that Fujifilm X-H2'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-H2 and S2 sensor size comparison.
As seen above, Leica S2 has a 3.7x Larger sensor area than Fujifilm X-H2. 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.