Both Canon S100 and Canon SX270 HS have 12.0 MP resolution sensors but Canon S100's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Canon SX270 HS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon S100's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Canon SX270 HS.
Below you can see the S100 and SX270 HS sensor size comparison.
As seen above, Canon S100 has a 1.5x Larger sensor area than Canon SX270 HS. 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.