Pentax K-3 III Monochrome has a
26.0MP APS-C (23 x 15.5 mm ) sized CMOS sensor and features PRIME V processor. On the other hand, Sony A9 has a
24.0MP Full frame (35.6 x 23.8 mm ) sized Stacked BSI-CMOS sensor and features BIONZ X processor.
Another difference between these two cameras is that Pentax K-3 III Monochrome's sensor lacks anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the K-3 III Monochrome and A9 sensor size comparison.
As seen above, Sony A9 has a 2.4x Larger sensor area than Pentax K-3 III Monochrome. 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.