Have you isolated what times the log grows to determin what is making it grow? If not, you can get this information from the Default Trace. Chart the trend when autogrow operations affect your log. Establishing the timeline might help you figure out what is causing it to grow.

Same exact codebase on 2012 as was on 2005? Or have changes been made since the migration?

In your index maintenance job are you doing ONLINE rebuilds now where before you were doing OFFLINE? Are you doing most re-organizes or rebuilds?

It might be worth reposting this in the SQL 2012 General Forum with a pointer back to this thread just get more eyes on it. Some folks have stopped watching the 2005 Forums.