I'm not sure about the egg whites....they are a protein and therefore their place is structure, not fluffiness. I use a super fine  'cake flour' it's low protein and really finely milled compared to normal flour.  It makes all the difference in fluffiness IMO.

In saying that, if the low protein flour makes a difference, perhaps less egg whites (less protein) would help too? 

But isn't the fluffiness part of the structure?