Nikon L31 has a 16.0MP 1/2.3' (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Sony A900 has a 25.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Bionz processor.
Sony A900's sensor provides 9MP more than Nikon L31'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 L31 and A900 sensor size comparison.
As seen above, Sony A900 has a 30.7x Larger sensor area than Nikon L31. 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.