Sony FX6 has a
10.9MP Full frame (36 x 24 mm ) sized CMOS sensor . On the other hand, Sony FX9 has a
19.0MP Full frame (35.7 x 18.8 mm ) sized CMOS sensor .
Sony FX9's sensor provides 8.1MP more than Sony FX6'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 FX6 and FX9 sensor size comparison.
Sensor Size and Resolution Comparison image of Sony FX6 and Sony FX9 Cameras
As seen above, Sony FX6 has a 1.3x Larger sensor area than Sony FX9. 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.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Sony FX6
or the
Sony FX9 ,
including hyperfocal distance.