I've said this for years - all adults allowed, say, 110kg total (bodyweight plus luggage) in their fare. Pay extra for any extra. Simple, and fair.
So somebody that is very tall & will be particularly uncomfortable in airline seats anyway, should pay more than the shorty's who the seats are designed for then? I think not
Weight costs fuel - tallness, in itself, doesn't.
We went to Mexico years ago, and repeatedly over the course of our stay, the rep kept going on about not overloading on the way back, because the airport staff were draconian.
When the time came to leave, we went to the airport, and stood in line. Turned out they didn't have weighing facilities at all.
So, being that way inclined (awkward), I asked the rep who was flitting around while we queued, how could they know that the plane had enough fuel to get back, if they didn't know how much luggage was on it.
Ahhhh - they'll just fill her up to the brim, she said.
I then pointed out that the flight back was 10 1/2 hrs, and the fully-laden plane's range was 11 1/2 hrs. And fuel weighs something, so you need fuel just to carry fuel.
I know they build in tolerance, and as it happened, we didn't ditch off Ireland, but there would surely come a point where too much luggage would shorten the range enough (especially if the pilots didn't actually know how much the plane's take-off weight was), that the plane couldn't get there....