The benefits of small product backlog items are well understood. Small product backlog items:

• are more likely to fit within an iteration
• allow team members to work concurrently on a few (but not too many!) items
• lead to a more consistent velocity when an item or two is occasionally left unfinished ​

But these advantages do not come without costs. One problem with small product backlog items is that it's possible for the team to lose sight of why they are doing those items.