First time I've ever heard anybody complain about a viewfinder being too big.
But, as others have suggested, it may be that the problem is the eye relief. I can't be bothered to compare the specs, but I wouldn't be surprised if the 6D has a comparatively close eye relief, meaning you have to mash your nose against the camera to get your eye close enough to see everything.
What I personally really want is the viewfinder from my parents's Pentax ME-Super on my 5DIII. Such an awesome viewfinder! Why, oh why, are we stuck with these small, dim, flat viewfinders these days? And let's not forget the wonderfulness of the split microprism...so much superior to a flashing red dot it's not even funny.
Ah, well...at least we do have live view, which more than makes up for it when shooting things that don't move.