Both Canon 1000D and Canon SX1 IS have 10.0 MP resolution sensors but Canon 1000D's sensor is APS-C (22.2 x 14.8 mm ) and Canon SX1 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon 1000D'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 SX1 IS.
Below you can see the 1000D and SX1 IS sensor size comparison.
As seen above, Canon 1000D has a 11.7x Larger sensor area than Canon SX1 IS. 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.