Both Sigma Quattro H and Canon R5 have 45.0 MP resolution sensors but Sigma Quattro H's sensor is APS-H (26.6 x 17.9 mm ) and Canon R5's sensor is Full frame (36 x 24 mm ).
Since Canon R5's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Sigma Quattro H.
Another difference between these two cameras is that Sigma Quattro H'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 Quattro H and R5 sensor size comparison.
As seen above, Canon R5 has a 1.8x Larger sensor area than Sigma Quattro H. 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.