Snape, deep down a massive who was always angry at Harry BECAUSE Harry was too thin, claims to have an antidote. Snape gets said "antidote" and gives it to Harry in small doses while rubbing Harry's belly. The potion actually starts to make him get slowly fatter and fatter. Snape throws the rest of the potion down his throat making him grow very rapidly. Before you know it, Harry's too big to move on his own, so snape uses the Wingadium Leviosa spell (pardon spelling) to move Harry into a large secret room where Snape feeds him for many long years to come O_O ... ... just a thought.