well my 500mm 5.6 for MF already has a filtter size of 110mm
so i am not suprised if its any larger. It should be otherwise it would be strange!
if anything a 600mm f 2.8 would end up to be a 400mm 2.8 in 35mm terms, due to the negetive crop factor, so in that sense its not very unrealistic.
but it would be insane for the photographer to lung around a monster like that. However if the price is right, i would be more than willing to do so!