Sony RX10 III has a 20.0MP 1" (13.2 x 8.8 mm ) sized BSI-CMOS sensor and features Bionz X processor. On the other hand, Sony A7 has a 24.0MP Full frame (35.8 x 23.9 mm ) sized CMOS sensor and features Bionz X processor.
Below you can see the RX10 III and A7 sensor size comparison.
As seen above, Sony A7 has a 7.4x Larger sensor area than Sony RX10 III. Larger sensors give photographer more control on the depth of field and blurry background compared to smaller sensor when shot in same focal length and aperture.