My ridiculous theory to go along with this ridiculous manga...
Shiranui's loli body is not her real body. She's using some form of high level transformation magic to disguise herself. That's why she eats such large amounts of food to keep up her guise. Tiring of her situation she reverts back to her old form.(see below)
She snags Zenkichi away from Medaka and starts a reign of terror against the Student Council...muwahahahahaha