Fujifilm XP90 has a 16.0MP 1/2.3" (6.17 x 4.55 mm ) sized BSI-CMOS sensor . On the other hand, Nikon 1 S1 has a 10.0MP 1" (13.2 x 8.8 mm ) sized CMOS sensor . Fujifilm XP90's sensor provides 6MP more than Nikon 1 S1'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 XP90 and 1 S1 sensor size comparison.
As seen above, Nikon 1 S1 has a 4.1x Larger sensor area than Fujifilm XP90. 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.