Fuji created their own equation named “Value Angle” to measure the quality of a mount. In short: The Greater the value angle, the more precise and easier a lens can send light to the sensor. A wider value angle gives more flexibility for lens design and allows for more light and less digital correction.
The way Fuji measures it the Sony E-mount does fine for APS-C while it ends up last for Full Frame:
Now keep in mind this isn’t an industry standard way to measure things. It’s “only” Fuji’s way to measure a mount.
via Profitoto via Fujirumors