Sony A900 has a
25.0MP Full frame (35.9 x 24 mm ) sized CMOS sensor and features Bionz processor. On the other hand, Pentax K-3 III has a
26.0MP APS-C (23 x 15.5 mm ) sized CMOS sensor and features PRIME V processor.
One other difference between these two cameras that is worth mentioning is that Pentax K-3 III's sensor doesn't have an 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 A900 and K-3 III sensor size comparison.
As seen above, Sony A900 has a 2.4x Larger sensor area than Pentax K-3 III. 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.