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Inverse Square law have something to do with it??
To get f/9 @ 600mm you need a wider aperture to have same light gathering ability as a 200mm f/9, but at a higher magnification. Higher quality components are needed to make the field sharp and free of vignetting @ f/2.8.
My guess is that they use a much bigger lens so that the manufacturer can control the vignetting in the lens, much like a field stop in an eyepiece.
I'd have to look into that a bit more i think to make sure!
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