Both Nikon D7000 and Nikon S3300 have 16.0 MP resolution sensors but Nikon D7000's sensor is APS-C (23.6 x 15.7 mm ) and Nikon S3300's sensor is 1/2.3-inch (6.17 x 4.55 mm ).
Since Nikon D7000'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 Nikon S3300.
Below you can see the D7000 and S3300 sensor size comparison.
As seen above, Nikon D7000 has a 13.2x Larger sensor area than Nikon S3300. 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.