There is no reason why bigger letters would mean that a voice is louder though it has certainly become convention in comics.

Catherine Khordoc
  • in <a href="http://www.amazon.com/gp/product/1578064147?ie=UTF8&tag=quooncom-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=1578064147" target="blank"><em>The Language of Comics: Word and Image</em></a><img src="http://www.assoc-amazon.com/e/ir?t=quooncom-20&l=as2&o=1&a=1578064147" width="1" height="1" border="0" alt="" style="border:none !important; margin:0px !important;" />
Other quotes from Catherine Khordoc.