Panasonic FZ200 has a 12.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features Venus Engine VII FHD 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.
Panasonic FZ1000's sensor provides 8MP more than Panasonic FZ200's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the FZ200 and FZ1000 sensor size comparison.
As seen above, Panasonic FZ1000 has a 4.1x Larger sensor area than Panasonic FZ200. 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.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It 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. Panasonic FZ200 and Panasonic FZ1000 sensors have been tested by DxO and the results show that FZ1000 has a better overall score of 64, 27 points higher compared to FZ200's score of 37.