Both Panasonic S5 and Pentax K-3 have 24.0 MP resolution sensors but Panasonic S5's sensor is Full frame (35.6 x 23.8 mm ) and Pentax K-3's sensor is APS-C (23.5 x 15.6 mm ).
Since Panasonic S5'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 Pentax K-3.
Another similarity between these two cameras is that both Pentax K-3 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 K-3 sensor size comparison.
As seen above, Panasonic S5 has a 2.3x Larger sensor area than Pentax K-3. 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.