It can't just be to compensate for less light gathering, can it?
(I mean, that's what the lesser weight does!) 
How come that a Xx30 bino of the same line (almost always, what I've seen) have larger F O V than a Xx40, or even more than a Xx50?
It can't just be to compensate for less light gathering, can it? (I mean, that's what the lesser weight does!)
The 30mm will usually have a shorter focal length and/or wider APPARENT FoV.
cheers,
Rick
Thanks Rick, but this leaves me with new questions! 
I can see why higher magnification gives less F o V, and longer focal length gives more magnification(?). But the magnification must be the same in a pair of 8x30 and 8x40 binoculars... or? What do I miss?
And apparent F o V, that has to do with the beam leaving the oculars, I think...
Maybe I just have to go back to wikipedia and study some more! 