Sony A7 III has a 24.0MP Full frame
(35.8 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Sony A7S III has a 12.0MP Full frame
(35.6 x 23.8 mm ) sized BSI-CMOS sensor and features Bionz XR processor.
Sony A7 III's sensor provides 12MP more than Sony A7S III's sensor, which gives a significant advantage in real life. You can print your images larger or crop more freely.
On the other hand, please keep in mind that Max sensor resolution is not the only determinant of resolving power. Factors such as the optical elements, low pass filter, pixel size and sensor technology also affects the final resolution of the captured image.
Below you can see the A7 III and A7S III sensor size comparison.
Sony A7 III and Sony A7S 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.
DxOMark Sensor Scores
Both Sony A7 III and Sony A7S III sensors have been tested by DxoMark. DxoMark scores camera sensors for color depth (DXO Portrait), dynamic range (DXO Landscape) and low-light sensitivity (DXO Sports), and also gives them an overall score. Of the two cameras that we are comparing, A7 III has scored 96, 11 points higher than A7S III.
Sony A7 III
Sony A7S III