Both Sigma fp L and Sony RX1R III have Full frame sized 61.0 MP resolution sensors so sensor size and resolution is not a differentiator between these two cameras.
One other difference between these two cameras that is worth mentioning is that Sony RX1R III'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 fp L and RX1R III sensor size comparison.
Sensor Size and Resolution Comparison image of Sigma fp L and Sony RX1R III Cameras
Sigma fp L and Sony RX1R III 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.
Sensor size sets the circle of confusion, so it drives how much depth of field each
of these bodies gives you at a given aperture. You can calculate it exactly for the
Sigma fp L
or the
Sony RX1R III ,
including hyperfocal distance.
