Casio EX-FC100 has a 9.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor . On the other hand, Casio EX-ZR800 has a 16.0MP 1/2.3" (6.17 x 4.55 mm ) sized CMOS sensor and features EXILIM Engine HS 3 processor.
Casio EX-ZR800's sensor provides 7MP more than Casio EX-FC100'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 EX-FC100 and EX-ZR800 sensor size comparison.
Casio EX-FC100 and Casio EX-ZR800 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 Casio EX-FC100 has 77% larger pixel area compared to Casio EX-ZR800, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.