title: Compressing data structure in flatbuffer increases file size?
body: <p>I was trying to optimize my flatbuffer size by creating a compressed schema     for the most repetitive struct I have (using only the 20 LSB bits of ints and slicing them into 64 bits words), the result is not what I was expecting: the file size grew a bit instead of the drastic compression I was expecting, why my 2 ulong + 1 uint struct (2nd case) below takes more space than the explicit struct (1rst case) in the file generated?
  In my example, a 2.4Mb file using the explicit struct became a 2.6M using the compressed one....</p>
  
  <p>Explicit Struct, direct access to fields:</p>
  
  <pre><code>table VObject {
      m_field1:uint; 
      m_field2:uint;
      m_field3:ushort;
      m_field4:ushort;
      m_field5:uint;
      m_field6:uint;
      m_field7:uint;
      m_field8:uint;
  }  
  </code></pre>
  
  <p>Compressed Struct, need encoding and decoding using 20 and 16 bit masks: </p>
  
  <pre><code>table VObject {
      m_field1:ulong; 
      m_field2:ulong;
      m_field3:uint;
  }
  </code></pre>