Fujirumors spotted an explanation from Fuji why they don’t go Full Frame. Fuji Manager Takashi Ueno says:
We gave a lot of thought to this, and reached our conclusion that APS-C is the best format for the optimum balance of body size and image quality.
Higher image quality can be achieved with full frame sensors, but in order to maximize the use of the sensor size, the lens will be very bulky and heavy.
With the power of FUJINON lens, we can achieve the full frame image quality with the APS-C sensor. We learned that the disadvantage can be recovered with the lens performance through various simulations.
Here is an example. It is generally believed that the lens performs at its best with 1 to 2 stop down from the maximum aperture. We tried to break the norm. Because if there is such belief, then the lens is very unlikely to perform at its best from corner to corner with the aperture wide open. But if we can break the norm, then we will achieve bokeh and sharpness that is equivalent to that of a full frame with 1 to 2 stop down. We can achieve the image quality that is equivalent to that of full frame.
Which is better? An f/1.4 lens on a full frame sensor and then used 1 stop down to prevent degradation in the image quality in the corner or an f/1.4 lens on an APS-C that see no degration in the image quality at its maximum aperture value?
You cannot really see the difference in bokeh between the APS-C wide open and the full frame one stop down. However, if the APS-C is wide open, then the shutter speed will be twice as fast as the full frame resulting in be less blur caused by hand shake or subject movement. If the picture becomes blurry, then the high image quality becomes meaningless.”
I have to say I do NOT agree with Fuji. Don’t think their X-Trans sensors can keep up with the Sony sensor technology madness on cameras like the A7s and A7rII. They definitely should go Full Frame or even Medium Format if the don’t want to remain in the very niche. Full Frame is already becoming mainstream with cameras like the A7 selling for less than $1,000!