Below you can see the front-view size comparison of the Nikon D3S and Fujifilm X-T1 IR. The Fujifilm X-T1 IR is clearly the smaller of the two cameras. Its body is 31 mm narrower, 67 mm shorter and 41 mm thinner than the Nikon D3S.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.