Panasonic FZ1000 II has a 20.0MP 1-inch (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Venus Engine processor. On the other hand, Ricoh GR III has a 24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor .
One other difference between these two cameras that is worth mentioning is that Ricoh GR III's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the FZ1000 II and GR III sensor size comparison.
As seen above, Ricoh GR III has a 3.2x Larger sensor area than Panasonic FZ1000 II. 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.