Seeing Nino become the fattest hog of the 5 is appealing because it fits with her character that she would be the most pissed off about becoming that way. Itsuki is more of a natural glutton so that is her appeal. And Miku seems to be the least superficial about appearance and is interested in niche indoor activities that would make it easier to get chubby without trying. Although Yotsuba quitting sports and getting fat would be cool and Ichika having to become some sort of bbw model/actress would work too. Yotsuba and Ichika were my least favorites in the show though so I'm glad they don't get the most art.