Both Sony HX20V and Sony HX90V have 18.0 MP resolution sensors but Sony HX20V's sensor is 1/2.3-inch (6.17 x 4.55 mm ) and Sony HX90V's sensor is (1/2. ).
Since Sony HX20V'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 HX20V and HX90V sensor size comparison.
As seen above, Sony HX20V 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.