That is, for any GET, you can add the "fields" query parameter to request that certain fields of the object also be returned. By default, only key fields are returned (fields that identify the record) when you query a collection. This is in contrast to the default behavior of an instance query (a request to a path that includes the identifier for a single record). For those requests, all fields are returned by default which is the behavior you saw.
Appending fields=* means that you want all the fields of the object (except for certain expensive to compute fields, to include those you would need fields=**). You could alternatively (and more efficiently) use fields=files if you only cared about the hard_limit and soft_limit, for example.
For more examples, look in the section of the REST documentation called "Requesting Specific Fields" which is near the top.