Both Olympus XZ-2 iHS and Canon SX280 HS have 12.0 MP resolution sensors but Olympus XZ-2 iHS's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Canon SX280 HS's sensor is 1/2.3-inch (6.17 x 4.55 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 Canon SX280 HS.
Below you can see the XZ-2 iHS and SX280 HS sensor size comparison.
As seen above, Olympus XZ-2 iHS has a 1.5x Larger sensor area than Canon SX280 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.