Both Nikon P340 and Leica X1 have 12.0 MP resolution sensors but Nikon P340's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Leica X1's sensor is APS-C (23.6 x 15.8 mm ).
Since Leica X1'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 Nikon P340.
Below you can see the P340 and X1 sensor size comparison.
As seen above, Leica X1 has a 9.0x Larger sensor area than Nikon P340. 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.