Canon 1D has a
4.0MP APS-H (28.7 x 19.1 mm ) sized CCD sensor . On the other hand, Canon 20D has a
8.0MP APS-C (22.5 x 15 mm ) sized CMOS sensor .
Below you can see the 1D and 20D sensor size comparison.
As seen above, Canon 1D has a 1.6x Larger sensor area than Canon 20D. 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.