Could my spotlight search index have really been 2.5 GB on a 256 GB drive?
All that you write sounds perfectly normal to me.


  
    does it all somehow get indexed?
  


Yes it does.


  
    Is there some magic way to stop it from opening file types I'm not interested in indexing?
  


Well, not directly. But you can exclude folders, by either adding .noindex to the folder name, or by going into System Preferences → Spotlight → Privacy. You may want to move all the files you don’t want indexed into such a location.

Also see How can I make spotlight ignore certain file types and directories? As far as I know there’s no way to tell Spotlight NOT to index certain file types.