Both Fujifilm X100S and Fujifilm X70 have 16.0 MP resolution sensors but Fujifilm X100S's sensor is APS-C (23.6 x 15.8 mm ) and Fujifilm X70's sensor is APS-C (23.6 x 15.6 mm ).
Since Fujifilm X100S's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Fujifilm X70.
Another difference between these two cameras is that Fujifilm X100S'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 moiré occurring in certain scenes.
Below you can see the X100S and X70 sensor size comparison.
Fujifilm X100S and Fujifilm X70 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.