Canon 400D has a 10.0MP APS-C
(22.2 x 14.8 mm ) sized CMOS sensor . On the other hand, Leica X Vario has a 16.0MP APS-C
(23.6 x 15.8 mm ) sized CMOS sensor .
Leica X Vario's sensor provides 6MP more than Canon 400D's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the 400D and X Vario sensor size comparison.
As seen above, Leica X Vario has a 1.1x Larger sensor area than Canon 400D. 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Canon 400D and Leica X Vario sensors have been tested by DxO and the results show that X Vario has a better overall score of 78, 16 points higher compared to 400D's score of 62.
Leica X Vario