I think you're looking at it upside down. It's about the perpetrators. Rape could be declining as obesity is rising because it's very difficult to be fat and a rapist. If you are a male fat would-be rapist, your penis wouldn't be able to get out of the folds to commit a rape with. Fat rapists of either indiscernible gender are thwarted from committing rape because they are too out of shape and the potential victim can just run away.