Both Olympus E-620 and FujiFilm S2500HD have 12.0 MP resolution sensors but Olympus E-620's sensor is Four Thirds (17.3 x 13 mm ) and FujiFilm S2500HD's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Olympus E-620'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 FujiFilm S2500HD.
Below you can see the E-620 and S2500HD sensor size comparison.
As seen above, Olympus E-620 has a 8.0x Larger sensor area than FujiFilm S2500HD. 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.