DiseaseOntology -- too big to be useable?

I found a number of open source ontologies that are being developed.  I wanted to get a feel for how far the idea has come.  This one stood out -- I stopped reviewing it after the browser had loaded over 500k lines.

It begs the question - how big is too big when it comes to the useability of an Ontology? 

Warning - this link may freeze your browser - http://svn.sourceforge.net/viewvc/diseaseontology/trunk/HumanDO.obo?view...