Both Canon SX720 HS and Canon G9 X have 20.0 MP resolution sensors but Canon SX720 HS's sensor is 1/2.3" (6.17 x 4.55 mm ) and Canon G9 X's sensor is 1 (13.2 x 8.8 mm ).
Since Canon G9 X'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 Canon SX720 HS.
Below you can see the SX720 HS and G9 X sensor size comparison.
As seen above, Canon G9 X has a 4.1x Larger sensor area than Canon SX720 HS. 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.