Both Canon SX540 HS and Canon G3 X have 20.0 MP resolution sensors but Canon SX540 HS's sensor is 1/2.3' (6.17 x 4.55 mm ) and Canon G3 X's sensor is 1' (13.2 x 8.8 mm ).
Since Canon G3 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 SX540 HS.
Below you can see the SX540 HS and G3 X sensor size comparison.
As seen above, Canon G3 X has a 4.1x Larger sensor area than Canon SX540 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.