Items on the bottom of the Product Backlog will not be developed soon, therefore, there's no need to spend time breaking them down into smaller items. When they move to the top of the Product Backlog, they are usually made smaller and
more detail will be added to them. Therefore, the items on the top of the Product Backlog are usually smaller than those at the bottom. Since the Sprint Backlog items come from the top of the Product Backlog, they would be smaller
than the average size of Product Backlog items.