On a narrow body they can't really add an extra seat across (you know they'd do it if they could). However, they could come up with a set of three seats where one is extra wide (for the larger folks out there), one is extra narrow (suitable for a kid), and one is normal. I'm sure they could work out a price structure that would put more cash in their pockets.
One thing the airlines might have going for them is the perception that obesity, unlike height, is something that a person can control. Therefore, if they are overflowing into an adjacent seat then it's their own fault and they should be the ones to pay the extra cash, not force "regular" people to accommodate their lack of willpower.