Fujifilm X-T30 has a 26.0MP APS-C (23.5 x 15.6 mm ) sized BSI-CMOS sensor and features X-Processor 4 processor. On the other hand, Fujifilm X-T20 has a 24.0MP APS-C (23.6 x 15.6 mm ) sized CMOS X-TRANS III sensor and features X-Processor Pro2 processor.
Another similarity between these two cameras is that both Fujifilm X-T20 and Fujifilm X-T30 sensors lack anti-alias (Low-Pass) filters. 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 X-T30 and X-T20 sensor size comparison.
Fujifilm X-T30 and Fujifilm X-T20 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.