Fujifilm X-Pro1 has a
16.0MP APS-C (23.6 x 15.6 mm ) sized CMOS X-TRANS I sensor and features EXR Pro processor. On the other hand, Canon 450D has a
12.0MP APS-C (22.2 x 14.8 mm ) sized CMOS sensor .
Another difference between these two cameras is that Fujifilm X-Pro1'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 moiré occurring in certain scenes.
Below you can see the X-Pro1 and 450D sensor size comparison.
As seen above, Fujifilm X-Pro1 has a 1.1x Larger sensor area than Canon 450D. 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.