title: What are the sizes returned by `show collections`?
body: <p><strong>Edit</strong>: This question is not about vanilla MongoDB's <code>show collections</code> but about <code>mongo-hacker</code>. See accepted answer and comments.</p>
  
  <hr>
  
  <p>Using Mongo DB 3.2 + WiredTiger, <code>show collections</code> displays two sizes: s1 / s2.</p>
  
  <pre><code>show collections
  coll_1               → 10.361MB / 1.289MB
  coll_2               →  0.000MB / 0.004MB
  coll_3               →  0.000MB / 0.016MB
  coll_4               →  0.001MB / 0.031MB
  </code></pre>
  
  <p>My guess is these are:</p>
  
  <ul>
  <li>s1: total size of the documents in the database</li>
  <li>s2: size of the database on disk (documents + indexes) after compression</li>
  </ul>
  
  <p>Is this correct? I couldn't find any reference in the <a href="https://docs.mongodb.com/manual/reference/mongo-shell/#command-helpers" rel="noreferrer">docs</a>.</p>