Both Sony HX90V and Sony WX500 have 18.0 MP resolution sensors but Sony HX90V's sensor is (1/2. ) and Sony WX500's sensor is 1/2.3 (6.17 x 4.55 mm ).
Since Sony WX500'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 HX90V and WX500 sensor size comparison.
As seen above, Sony WX500 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.