Both Panasonic FS12 and Canon 450D have 12.0 MP resolution sensors but Panasonic FS12's sensor is 1/2.3-inch (6.08 x 4.56 mm ) and Canon 450D's sensor is APS-C (22.2 x 14.8 mm ).
Since Canon 450D'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 Panasonic FS12.
Below you can see the FS12 and 450D sensor size comparison.
As seen above, Canon 450D has a 11.9x Larger sensor area than Panasonic FS12. 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.