Leica S2 has a 38.0MP Medium format (45 x 30 mm ) sized CCD sensor . On the other hand, Sony A7R II has a 42.0MP Full frame (35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor.
One other difference between these two cameras that is worth mentioning is that Sony A7R II'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 S2 and A7R II sensor size comparison.
As seen above, Leica S2 has a 1.6x Larger sensor area than Sony A7R II. 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Leica S2 and Sony A7R II sensors have been tested by DxO and the results show that A7R II has a better overall score of 98, 22 points higher compared to S2's score of 76.