Both Panasonic FZ2500 and Panasonic FZ1000 have 20.0 MP resolution sensors but Panasonic FZ2500's sensor is 1" (13.2 x 8.8 mm ) and Panasonic FZ1000's sensor is 1 (13.2 x 8.8 mm ).
Since Panasonic FZ1000's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Panasonic FZ2500.
Below you can see the FZ2500 and FZ1000 sensor size comparison.
Panasonic FZ2500 and Panasonic FZ1000 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
Both Panasonic FZ2500 and Panasonic FZ1000 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, FZ2500 has scored 70, 6 points higher than FZ1000.