Fujifilm S1 Pro has a 3.0MP APS-C (23 x 15.5 mm ) sized CCD sensor . On the other hand, Fujifilm S2 Pro has a 6.0MP APS-C (23 x 15.5 mm ) sized CCD sensor .
Below you can see the S1 Pro and S2 Pro sensor size comparison.
Fujifilm S1 Pro and Fujifilm S2 Pro have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Fujifilm S1 Pro has 97% larger pixel area compared to Fujifilm S2 Pro, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.