title: Mongodb data files become smaller after migration
body: <p>On my first server I get:</p>
  
  <pre><code>root@prod ~ # du -hs /var/lib/mongodb/
  909G    /var/lib/mongodb/
  </code></pre>
  
  <p>After migration <strong>this</strong> database with mongodump/mongorestore
  On my second server I get:</p>
  
  <pre><code>root@prod ~ # du -hs /var/lib/mongodb/
  30G /var/lib/mongodb/
  </code></pre>
  
  <p>After I waited a few hours, mongo finished indexing I got:</p>
  
  <pre><code>root@prod ~ # du -hs /var/lib/mongodb/
  54G /var/lib/mongodb/
  </code></pre>
  
  <p>I tested database and there's no corrupted or missed data.</p>
  
  <p>Why there's so big difference in size before and after migration?</p>