Both Nikon D5200 and Fujifilm X-T20 have 24.0 MP resolution sensors but Nikon D5200's sensor is APS-C (23.5 x 15.6 mm ) and Fujifilm X-T20's sensor is APS-C (23.6 x 15.6 mm ).
Since Fujifilm X-T20's has a larger sensor area with the same resolution, this means that it also has a larger pixel area hence better light collecting capacity for a given aperture compared to Nikon D5200.
One other difference between these two cameras that is worth mentioning is that Fujifilm X-T20'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 moiré occurring in certain scenes.
Below you can see the D5200 and X-T20 sensor size comparison.
Nikon D5200 and Fujifilm X-T20 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.