Meg

A Meg weight gain episode practically writes itself. She's so disliked by everyone, and is so correspondingly desperate for affection, that you can easily picture herself latching on to the pathetic devotions of fat fetishists. Maybe she gets a little chubby and someone in the community takes a shine to her. Maybe she's browsing the internet one day and stumbles onto a website.
Either way, in no time at all she'd realize that weight gain=affection for these people, so she'd start to fatten herself up. And the fatter she got, the more she'd be worshiped by the disgusting degenerates of the community. She could wind up confined to a bed with an oxygen tank, and she'd still be having the time of her life, because, at last, people love her.
Also I can see Meg turning her hobby of being an internet BBW into a career and starts making a ton of money selling photo and video collection sets and making commissions.
Lois want to put and end to this and has Peter talk to her but when Peter sees how much money Meg is making he decides to help her and basically becomes her feeder. It'd make the whole situation even more pathetic for Meg since she's doing this to herself not only for the praise of people she doesn't even know but because her father finally accepts her, but only in a twisted and manipulative sort of way.
Lois becomes even fatter just to prove to Meg they arent really friends, admirers, lovers but are just a bunch of weirdos who will go for the fattest thing.