Nikon S8200 has a
16.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor. On the other hand, Nikon S8100 has a
12.0MP 1/2.3-inch (6.17 x 4.55 mm ) sized BSI-CMOS sensor and features Expeed C2 processor.
Below you can see the S8200 and S8100 sensor size comparison.
Nikon S8200 and Nikon S8100 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. On the other hand, since Nikon S8100 has 32% larger pixel area (2.34µm2 vs 1.76µm2) compared to Nikon S8200, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.