Both Fujifilm F900EXR and Panasonic ZS45 have 16.0 MP resolution sensors but Fujifilm F900EXR's sensor is 1/2-inch (6.4 x 4.8 mm ) and Panasonic ZS45's sensor is 1/2.33 (6.08 x 4.56 mm ).
Since Fujifilm F900EXR'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 ZS45.
Below you can see the F900EXR and ZS45 sensor size comparison.
As seen above, Fujifilm F900EXR has a 1.1x Larger sensor area than Panasonic ZS45. 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.