Dynamodb documentation [1] clearly states that:

• “Item size” cannot exceed 400KB.
• “Number of elements in a list”: An attribute of type List can contain more than two billion elements.

I must be misunderstanding something here, if you have 2 billion items in a list attribute, then the item containing this attribute is surely larger than 400KB, right?

What am I missing?

1- http://docs.aws.amazon.com/amazondynamodb/latest/developerguide/Limits.html

You are correct, the number of elements in a List Attribute Value will be limited to the size of the item, which is currently 400KB. Thanks for the feedback on the documentation, we will be sure to incorporate this, thanks!