Sony A99 II has a 42.0MP Full frame (35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Pentax K-1 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor .
Sony A99 II's sensor provides 6MP more than Pentax K-1'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 K-1 and Sony A99 II 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 A99 II and K-1 sensor size comparison.
Sony A99 II and Pentax K-1 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 Pentax K-1 has 16% larger pixel area (23.83µm2 vs 20.43µm2) compared to Sony A99 II, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.
DxOMark Sensor Scores
DxOMark is a benchmark that scientifically assesses image quality of camera sensors. It scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Sony A99 II and Pentax K-1 sensors have been tested by DxO and the results show that K-1 has a better overall score of 96, 4 points higher compared to A99 II's score of 92.