Both Sony A57 and Olympus E-M1 have 16.0 MP resolution sensors but Sony A57's sensor is APS-C (23.5 x 15.6 mm ) and Olympus E-M1's sensor is Four Thirds (17.3 x 13 mm ).
Since Sony A57'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 Olympus E-M1.
Below you can see the A57 and E-M1 sensor size comparison.
As seen above, Sony A57 has a 1.6x Larger sensor area than Olympus E-M1. 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.