Below you can see the front-view size comparison of the Fujifilm X-T1 IR and Pentax K-3. The Fujifilm X-T1 IR is clearly the smaller of the two cameras. Its body is 2mm narrower, 10 mm shorter and 30 mm thinner than the Pentax K-3.
The chart below depicts how two cameras compare with average dimensions of the cameras in their class.