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