Both Fujifilm X-T1 IR and Panasonic GH3 have 16.0 MP resolution sensors but Fujifilm X-T1 IR's sensor is APS-C (23.6 x 15.6 mm ) and Panasonic GH3's sensor is Four Thirds (17.3 x 13 mm ).
Since Fujifilm X-T1 IR'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 Panasonic GH3.
Another difference between these two cameras is that Fujifilm X-T1 IR'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-T1 IR and GH3 sensor size comparison.
As seen above, Fujifilm X-T1 IR has a 1.6x Larger sensor area than Panasonic GH3. 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.