Fujifilm X100 has a 12.0MP APS-C (23.6 x 15.8 mm ) sized CMOS sensor and features EXR processor. On the other hand, 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.
Another similarity between these two cameras is that both Fujifilm X100S and Fujifilm X100 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 X100 and X100S sensor size comparison.
Fujifilm X100 and Fujifilm X100S 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 compared to Fujifilm X100S, it has potential to collect more light on pixel level hence have less noise in low light / High ISO images.