Both Sony WX300 and Sony HX90V have 18.0 MP resolution sensors but Sony WX300's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Sony HX90V's sensor is (1/2. ).
Since Sony WX300'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 Sony HX90V.
Below you can see the WX300 and HX90V sensor size comparison.
As seen above, Sony WX300 has a infx Larger sensor area than Sony HX90V. 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.