Obese people would throw a conniption fit if something like this was implemented. But, it's a very sensible idea from the point of view of equality. I mean, nobody likes to sit next to someone that has rolls flowing over the armrests

. In essence, if someone is grossly overweight, they are decreasing the comfort of people sitting next to them immensely. The people sitting next to them should in fact, be charged less, and perhaps be compensated as well (with the extra money from the overweight person's ticket)?
But, since they're openly price discriminating based on traveller's physical attributes, I would also tend to wonder, what else could they implement? They should also charge more for parents that bring babies as well. I think that babies still get onto flights for free? Or have things changed?
What if the baby is well behaved and doesn't cry? What if it does? The airliner's can't tell these things beforehand. They can't tell your weight either, if you book and pay online. And surely they can't ask you to enter your weight, and then verify it when you fly.
It's very tricky waters for the airlines. They could risk a lot of negative publicity by following through with such plans. I wonder if anyone will implement this? I'd say, maybe in extreme circumstances, yes, if someone is really overweight, charge them for 2 seats (Like Southwest does). Otherwise, is it really worth hassling each customer by weighing them. I mean, women don't even want to tell people their age, and now you want them to be publicly weighed

!?!
From the point of view of petrol consumption, each additional person has a negligible effect on petrol consumption (yes I know petrol is going up, but in relation to the each fare charged, it's negligible). They need to weigh up the cost vs. benefit of weighing their customers before doing something like this.