Leica X-U has a 16.0MP APS-C (23.6 x 15.7 mm ) sized CMOS sensor . On the other hand, Leica Q has a 24.0MP Full frame (36 x 24 mm ) sized CMOS sensor and features Maestro II processor.
Leica Q's sensor provides 8MP more than Leica X-U'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 X-U and Q Typ 116 sensor size comparison.
As seen above, Leica Q has a 2.3x Larger sensor area than Leica X-U. 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.