Canon 550D has a 18.0MP APS-C (22.3 x 14.9 mm ) sized CMOS sensor and features Digic 4 processor. On the other hand, Pentax K-S2 has a 20.0MP APS-C (23.5 x 15.6 mm ) sized CMOS sensor and features PRIME MII processor.
One other difference between these two cameras that is worth mentioning is that Pentax K-S2'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 moire occurring in certain scenes.
Below you can see the 550D and K-S2 sensor size comparison.
As seen above, Pentax K-S2 has a 1.1x Larger sensor area than Canon 550D. 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.