Here's how it all goes down: the Trunchbull tries to force Bruce to eat a cake that's eighteen inches in diameter (as a punishment for swiping some of her cake earlier). It's bigger than his head, and no reasonable person would be able to eat more than a quarter of it. Probably. The Trunchbull is hoping that he'll eat until he's ill; he'll have too much of what he wanted, and not on his own terms.
But her plan fails in spectacular fashion. Bruce rises to the challenge, showing he has hidden depths—at least, as a competitive eater. Once he gets going, he keeps going. And going. And going.