Both Olympus E-PL6 and Sony NEX-5T have 16.0 MP resolution sensors but Olympus E-PL6's sensor is Four Thirds (17.3 x 13 mm ) and Sony NEX-5T's sensor is APS-C (23.4 x 15.6 mm ).
Since Sony NEX-5T'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-PL6.
Below you can see the E-PL6 and NEX-5T sensor size comparison.
As seen above, Sony NEX-5T has a 1.6x Larger sensor area than Olympus E-PL6. 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.