Oh, and I'm not suggesting this because I'm 6' 5" but fit and weigh less than most chunky medium height people I know... not at all.
How so? Taller people are also often heavier, simply due to the fact that they're bigger. And if the past they tended to be slimmer relative to their height, that's no longer true for male Americans: http://ajcn.nutrition.org/content/87/2/445.full
So taller people would still pay more than shorter, on average.
Someone's gender is genetic and men would pay more under your scheme as would taller people.
You may not weigh more than a very muscular or overweight person but by being longer you by definition take up more linear space.
Life has tradeoffs, you can't start correcting to make everything "fair". It's an impossible task, and not really worth anything in the end.
Alternatively, the airline could reserve them for tall people, but that risks leaving with them empty, which increases the costs for everyone.
I mean, maybe? The simple truth is that if you want to fly someone needs to pay more, because you weigh more and occupy more space in the vehicle. Is air travel something that should be subsidized in some way or not? I'm not sure that I see a clear answer here.
Alternatively: There are many financial inequalities that stem from genetics or circumstance. Is this the one we want to be fighting for?