Canon M6 has a 24.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 7 processor. On the other hand, Canon R has a 30.0MP Full frame (36 x 24 mm ) sized CMOS sensor .
Canon R's sensor provides 6MP more than Canon M6'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.
Below you can see the M6 and R sensor size comparison.
As seen above, Canon R has a 2.6x Larger sensor area than Canon M6. 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. Canon M6 and Canon R sensors have been tested by DxO and the results show that R has a better overall score of 89, 11 points higher compared to M6's score of 78.