I think the plane would have to be completely full of severely overweight people before it would affect fuel economy.
I do think if they are over a certain weight, they are likely to take up two seats so in THAT regard, yeah charging for two seats is kind of necessary.
I do not know, I mean even for normal sized humans, plane seats are awfully cramped.
Ain't like the movies where it looks like they are in a living room or theater.