[dylan604]

Seat configurations probably. They have a team of people that work on tweaking seating configs to allow more seats. Take away 1/2" per row for 100 rows, and you now have 50" of "new" space. Remove X inches in width per seat and Y inches from the aisle(s), and you suddenly get 1 additional set per row. I don't know what actual numbers are, but that's how they play with it.

[rbanffy]

There is a place in hell reserved for them.

Sadly, replacing the seats for ever thinner ones is much more costly than crushing my legs in economy. Or economy "plus", which is not much better.