Both Canon SD4500 IS and Nikon P7000 have 10.0 MP resolution sensors but Canon SD4500 IS's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Nikon P7000's sensor is 1/1.7-inch (7.44 x 5.58 mm ).
Since Nikon P7000'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 SD4500 IS.
Below you can see the SD4500 IS and P7000 sensor size comparison.
As seen above, Nikon P7000 has a 1.5x Larger sensor area than Canon SD4500 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.