Both Olympus E-PL9 and Olympus SZ-31MR iHS have 16.0 MP resolution sensors but Olympus E-PL9's sensor is Four Thirds (17.3 x 13 mm ) and Olympus SZ-31MR iHS's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Olympus E-PL9'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 SZ-31MR iHS.
Below you can see the E-PL9 and SZ-31MR iHS sensor size comparison.
As seen above, Olympus E-PL9 has a 8.0x Larger sensor area than Olympus SZ-31MR iHS. 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.