Hello John,
Cloud Datastore automatically scales its size up or down, you don't need to change anything.
The pie chart is showing the size for each resource (as shown in the table on its right) relative to the total size currently taken by this datastore. If you hover over the colored segments you will see to which resource the segment belongs. In other words, for your project, you will learn that almost all data is used by storing the actual entities (in blue), and that a much smaller part is used for the built-in indexes (that is, for indexed properties), in green. Composite indexes, in your case 0, refers to indexes that are composed of multiple indexed properties or ancestor queries.
You can drill down the chart / stats to certain namespaces or kinds. The documentation covers more information about
Datastore statistics and the
Dashboard.
Hope that clarified your question.