Originally posted by fuzz4brains
Well, I wouldn't say that the Fuji 100 Mp sensor is only using a fraction of the full image circle.
It's not a crop sensor considering it's a GFX 100 (44x33 mm sensor i.e. the same sensor size as a 645 sensor (645Z).
A 44x33 mm sensor has a diagonal of 55 mm, out of the 69,7 mm diagonal of a FF 645.
A crop sensor with a 1.4x TC you crop down the image to using an diagonal of 39.2 mm of the optics (approx 31.5 x 23.6 size).
So with crop 645 and 1.4x TC less than a third of the optics is used compared to a FF 645 (without TC).
(So it is something like in between APS-C and MFT, compared to 35mm FF).
So if a crop smaller than 35 mm FF on 645 optics can handle 100MP, there may be hope that good 35mm FF lenses may also support 100MP. and good APS-C lenses may be able to handle 45MP.