Fujifilm S2 Pro has a 6.0MP APS-C (23 x 15.5 mm ) sized CCD sensor . On the other hand, Fujifilm S1 Pro has a 3.0MP APS-C (23 x 15.5 mm ) sized CCD sensor .
Below you can see the S2 Pro and S1 Pro sensor size comparison.
Fujifilm S2 Pro and Fujifilm S1 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 (58.16µm2 vs 29.41µm2) compared to Fujifilm S2 Pro, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.