Fujifilm X100S has a 16.0MP APS-C (23.6 x 15.8 mm ) sized CMOS X-TRANS II sensor and features EXR II processor. On the other hand, Fujifilm X100 has a 12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features EXR processor.
Another similarity between these two cameras is that both Fujifilm X100 and Fujifilm X100S sensors lack anti-alias (Low-Pass) filters. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moire occurring in certain scenes.
Below you can see the X100S and X100 sensor size comparison.
Fujifilm X100S and Fujifilm X100 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 X100 has 30% larger pixel area (30.54µm2 vs 23.33µm2) compared to Fujifilm X100S, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.