Panasonic S5 has a 24.0MP Full frame (35.6 x 23.8 mm ) sized CMOS sensor and features Venus Engine processor. On the other hand, Pentax 645D has a 40.0MP Medium format (44 x 33 mm ) sized CCD sensor and features Prime II processor.
Pentax 645D's sensor provides 16MP more than Panasonic S5'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.
Another similarity between these two cameras is that both Pentax 645D and Panasonic S5 sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the S5 and 645D sensor size comparison.
![Panasonic S5 vs Pentax 645D Sensor Size Comparison]()
Sensor Size and Resolution Comparison image of Panasonic S5 and Pentax 645D Cameras
As seen above, Pentax 645D has a 1.7x Larger sensor area than Panasonic S5. 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.