Both Canon S90 and Canon S200 have 10.0 MP resolution sensors but Canon S90's sensor is 1/1.7-inch (7.44 x 5.58 mm ) and Canon S200's sensor is 1/1.7 (7.44 x 5.58 mm ).
Since Canon S200'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 S90.
Below you can see the S90 and S200 sensor size comparison.
Canon S90 and Canon S200 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.