"Extracting detail from an 8 power image is even easier, and, in objective tests conducted by Zeiss, birders consistently extracted the most detail (at least on eye charts) from a 7 power image. As noted above, my experience has been that there is no practical difference in the amount of detail you can see in hand held binoculars of equal quality between 7 and 10 power"
Source
http://betterviewdesired.com/BirdW.html
It's a classic article many should read before coming to any conclusions about magnification, objective size, brightness, etc.
John
That is, in fact, one of my favorite Mr. Ingraham quotes. The other being something to the effect of, if you cant see it in 7x, 10x isn't going to help either, you need a scope.
I think many times we find that the resolution and detail that high end binoculars deliver is much beyond our ability to discern. I do not doubt that there are those who love to see the bird bigger, myself preferring to see with more stability and context.
Personally, when I identify a bird I use shape, behavior, and habitat/time of year as much as or more than wingbars and supercilium stripes.