Olympus E-1 has a
5.0MP Four Thirds (17.3 x 13 mm ) sized CCD sensor . On the other hand, Nikon D100 has a
6.0MP APS-C (23.7 x 15.5 mm ) sized CCD sensor .
Below you can see the E-1 and D100 sensor size comparison.
As seen above, Nikon D100 has a 1.6x Larger sensor area than Olympus E-1. 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.