My experience with Ryanair is that they were about equally comfortable to an Alaska Air seat. My 6' son, with an aisle seat, thought Ryanair was fine in terms of comfort. I think the pitch is cramped, but the flights are all pretty short anyway.

Most passengers are not terribly fat or terribly tall. Most passengers are probably under 5'10, because after all there are an awful lot of passengers who are women, children, and men under 5'10. I think the airline comfortably meets the needs of at least 80% of its passengers with its current seats. I would guess another 19% can manage to squeeze in. Maybe 1% really, literally can't squeeze into their seats. (I've never seen this happen personally.) Using the Pareto principle, airlines are doing fine.

If we are going to charge by weight, there's no reason a 2-year-old's fare should be the same as that of a 400-pound, 6'6 samurai, but currently the two passengers pay exactly the same amount. This makes economic sense, though, because passengers would be driven away in droves if required to weigh in before boarding, not to mention the process would be very difficult and expensive to administer.