What makes you think it would need to be twice as big as a 1200mm, The Sigma can do 600mm in a lens a bit bigger than a 100-400mm ?? Ok so I did forget about the f2 part of my problem.. but thats up to the geniuses of the world... Not me. I just do the wanting
forum: Canon EF and EF-S Lenses
Exactly, it's not mostly (or even primarily) about focal length; it's about the size of the front element.
1200/5.6 = 214 (roughly)
800/2 = 400
The front element is almost twice as big across, which also means that much thicker and MUCH more than twice as heavy. Think of the difference between the 400/5.6 and the 400/2.8. That difference is two stops. An 800/2 is equivalent in front element to a 400/1, which is a whole three stops bigger than the 400/2.8.
I stand by my statement on the size of such a lens!