Both Sony S2100 and Olympus XZ-2 iHS have 12.0 MP resolution sensors but Sony S2100's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Olympus XZ-2 iHS's sensor is 1/1.7-inch (7.44 x 5.58 mm ).
Since Olympus XZ-2 iHS'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 Sony S2100.
Below you can see the S2100 and XZ-2 iHS sensor size comparison.
As seen above, Olympus XZ-2 iHS has a 1.5x Larger sensor area than Sony S2100. 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.