Sigma fp has a 25.0MP Full frame (35.9 x 23.9 mm ) sized BSI-CMOS sensor . On the other hand, Fujifilm X-T3 has a 26.0MP APS-C (23.5 x 15.6 mm ) sized X-Trans CMOS 4 sensor and features X-Processor 4 processor.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-T3'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 moire occurring in certain scenes.
Below you can see the fp and X-T3 sensor size comparison.
As seen above, Sigma fp has a 2.3x Larger sensor area than Fujifilm X-T3. 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.