I think this is a valid point, i.e. height is an immutable characteristic, whereas weight doesn't necessarily have to be (though I acknowledge that some people have medical conditions or genetic predispositions that make weight loss extremely difficult).

However, if you're going to make "choice" the distinguishing factor (assuming, again for the sake of argument that people choose their weight), then doesn't that apply to any "life style" choice? Does that mean that people who have chosen to have children should be required to sit in special sections or fly on special flights (I'd actually support that, but that's another thread)?

I see it as a question of special needs. I have a special need when I fly -- I'm mildly flying phobic (I used to be very, very flying phobic) and one thing that helps me is being able to look out the window. For this reason, I always book window seats (Always! Only 2 exceptions in 20+ years of frequent flying). It wouldn't occur to me to book an aisle and then tell the person seated next to me that they have to raise the window shade and lean back so that I can look out.

If, like me, someone has a special need, it's their responsibility to see that need is met, not the responsibility of other passengers.