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, Panasonic FZ1000 has a 20.0MP 1" (13.2 x 8.8 mm ) sized CMOS sensor and features Venus Engine processor.
Below you can see the P900 and FZ1000 sensor size comparison.
As seen above, Panasonic FZ1000 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.