StreamBuilder in flutter/sirestore implements pagination internally?

When using StreamBuilder with Firestore collection snapshots, does the Stream read ALL the documents of the collection the first time, or does it internally implement some sort of "throttling" mechanism to fetch limited number of documents at a time? I am building a chat app and have used StreamBuilder, I am worried that the first time a user loads the app, the stream will read thousands of messages, increasing my firestore bill.