Werner is right. The query operation is able to be more efficient than GetItem and BatchGetItems. To calculate how many units of read capacity will be consumed by a query, take the total size of all items combined and round up to the nearest whole KB. For example, if your query returns 10 items that were each 1KB, then you will consume 10 units of read capacity. If your query returns 10 items that were all 0.1KB, then you will consume only 1 unit of read capacity.

This is currently an undocumented benefit of the query operation, but we will be adding that to our documentation shortly.