The smallest child in the class spreads a rumor that his uncle is a giant, and when Uncle David comes to school for show-and-tell, the children learn more about their classmate's large relative.
It's a terrible thing to admit that sometimes you don't understand a picture book. I've read this a few times now, and I don't know that I get it yet. It's about tall stories, and how stories grow they as they are passed around. Ned boasts that his Uncle David is a giant, that he can lift six children at once. Then Uncle David in reality does seem to be an actual giant, living in a monastery like building, with multiple joints of meat in his fridge.