If you're bigger and thus need more food, you have to pay more at grocery stores and restaurants. I don't see how this is any different, nothing wrong with paying for how much you actually use.

But perhaps because you're bigger you get to eat more and then spend less on weight-loss products.

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.

If I'm paying the same for both seats, I might as well get a larger one, and then taller people can't even choose to pay more, they're just screwed.

Alternatively, the airline could reserve them for tall people, but that risks leaving with them empty, which increases the costs for everyone.

Only if your planes are fully booked on all trips. Otherwise you may losing revenues because of clients like me who have their own blacklist of airlines they don't want to fly with because of this sort of things. For instance I will not consider going to a destination which forces me to fly Ryanair.