Fujifilm X-T3 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, Sigma Quattro has a 29.0MP APS-C (23.5 x 15.6 mm ) sized CMOS (Foveon X3) sensor and features Dual TRUE III processor.
Another difference between these two cameras is that Fujifilm X-T3'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 moire occurring in certain scenes.
Below you can see the X-T3 and Quattro sensor size comparison.
Fujifilm X-T3 and Sigma Quattro have sensor sizes so they will provide same level of control over the depth of field when used with same focal length and aperture. On the other hand, since Sigma Quattro has 32% larger pixel area (18.69µm2 vs 14.12µm2) compared to Fujifilm X-T3, it has larger pixel area to collect light hence potential to have less noise in low light / High ISO images.