Both Nikon P340 and Canon ELPH 510 HS have 12.0 MP resolution sensors but Nikon P340's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Canon ELPH 510 HS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Nikon P340'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 ELPH 510 HS.
Below you can see the P340 and ELPH 510 HS sensor size comparison.
As seen above, Nikon P340 has a 1.5x Larger sensor area than Canon ELPH 510 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.