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 KP has a
24.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME IV processor.
Below you can see the A900 and KP sensor size comparison.
As seen above, Sony A900 has a 2.4x Larger sensor area than Pentax KP. 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.