Both Canon SX120 IS and Canon SD4500 IS have 10.0 MP resolution sensors but Canon SX120 IS's sensor is 1/2.5-inch (5.744 x 4.308 mm ) and Canon SD4500 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Canon SD4500 IS'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 SX120 IS.
Below you can see the SX120 IS and SD4500 IS sensor size comparison.
As seen above, Canon SD4500 IS has a 1.1x Larger sensor area than Canon SX120 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.