Nikon P900 has a 16.0MP 1/2.3 (6.17 x 4.55 mm ) sized CMOS sensor and features Expeed C2 processor. On the other hand, Canon G3 X has a 20.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features DIGIC 6 processor.
Below you can see the P900 and G3 X sensor size comparison.
As seen above, Canon G3 X has a 4.1x Larger sensor area than Nikon P900. 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.