Both Fujifilm X100 and Ricoh GXR GR Lens A12 28mm F2.5 have APS-C sized 12.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
Another difference between these two cameras is that Fujifilm X100's sensor lacks 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 moire occurring in certain scenes.
Below you can see the X100 and GXR GR Lens A12 28mm F2.5 sensor size comparison.
Fujifilm X100 and Ricoh GXR GR Lens A12 28mm F2.5 have almost the same sensor size so none of them has any significant advantage over other in providing control over depth of field when used with same focal length and aperture.