Sony A7R III has a 42.0MP Full frame
(35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Leica M9 has a 18.0MP Full frame
(36 x 24 mm ) sized CCD sensor .
Sony A7R III's sensor provides 24MP more than Leica M9's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Another similarity between these two cameras is that both Leica M9 and Sony A7R III sensors lack anti-alias (Low-Pass) filters. 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 A7R III and M9 sensor size comparison.
Sony A7R III and Leica M9 have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.
DxOMark Sensor Scores
Both Sony A7R III and Leica M9 sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, A7R III has scored 100, 31 points higher than M9.
Sony A7R III