Leica X1 has a
12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor . On the other hand, Leica M8 has a
10.0MP APS-H (27 x 18 mm ) sized CCD sensor .
One other difference between these two cameras that is worth mentioning is that Leica M8's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the X1 and M8 sensor size comparison.
As seen above, Leica M8 has a 1.3x Larger sensor area than Leica X1. 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.