Ricoh GXR GR Lens A12 28mm has a
12.0MP APS-C (23.6 x 15.7 mm ) sized CMOS sensor and features GR Engine III 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.
One other difference between these two cameras that is worth mentioning is that Fujifilm X100S's sensor doesn't have an anti-alias (Low-Pass) filter. Removing anti-alias filter increases the sharpness and level of detail but at the same time, it increases the chance of moiré occurring in certain scenes.
Below you can see the GXR GR Lens A12 28mm F2.5 and X100S sensor size comparison.
Ricoh GXR GR Lens A12 28mm and Fujifilm X100S have almost the same sensor size, so neither of them has any significant advantage over the other in terms of providing control over depth of field when used with the same focal length and aperture.