Because the fatty sees itself as thin.  Their minds are so clogged with beetus that their actual self image is that of a human. I've noticed this problem with many fatties trying to participate in cosplay.  They'll grab some show or movie and immediately try to emulate the most physically active character or sexy bombshell.  They have the body of Penguin but they'll insist on Harley Quinn.  I'm sure part of it is because they want to experience the attention that the fix characters receive, but don't make the obvious connection that fit characters are charismatic because they're healthy and hot.  Seven of Nine could dress like Rakk Hive and still have people drool over her.