Pentax K-1 has a 36.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor . On the other hand, Sony A99 II has a 42.0MP Full frame (35.9 x 24 mm ) sized BSI-CMOS sensor and features Bionz X processor.
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 Sony A99 II and Pentax K-1 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 K-1 and A99 II sensor size comparison.
Pentax K-1 and Sony A99 II 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 compared to Sony A99 II, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.
DxOMark Sensor Scores
Both Pentax K-1 and Sony A99 II sensors have been tested by DxoMark. DxoMark 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. Of the two cameras that we are comparing, K-1 has scored 96, 4 points higher than A99 II.