Both Canon S95 and Canon S90 have 10.0 MP resolution sensors but Canon S95's sensor is 1/1.7 (7.44 x 5.58 mm ) and Canon S90's sensor is 1/1.7" (7.44 x 5.58 mm ).
Since Canon S90'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 Canon S95.
Below you can see the S95 and S90 sensor size comparison.
Canon S95 and Canon S90 have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.
DxOMark Sensor Scores
Both Canon S95 and Canon S90 sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, S95 has scored 47, 1 points higher than S90.