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 FZ300 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features Venus Engine processor.
Below you can see the P900 and FZ300 sensor size comparison.
Nikon P900 and Panasonic FZ300 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Panasonic FZ300 has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Nikon P900, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.