Both Canon SX70 HS and Sony RX10 III have 20.0 MP resolution sensors but Canon SX70 HS's sensor is 1/2.3 (6.17 x 4.55 mm ) and Sony RX10 III's sensor is 1" (13.2 x 8.8 mm ).
Since Sony RX10 III'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 SX70 HS.
Below you can see the SX70 HS and RX10 III sensor size comparison.
As seen above, Sony RX10 III has a 4.1x Larger sensor area than Canon SX70 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.