Both Canon S200 and Canon S90 have 10.0 MP resolution sensors but Canon S200's sensor is 1/1.7 (7.44 x 5.58 mm ) and Canon S90's sensor is 1/1.7-inch (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 S200.
Below you can see the S200 and S90 sensor size comparison.
Canon S200 and Canon S90 have the same sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture.