Thanks for the help guys, but I found a solution on the sencha forum which seems to work:
http://www.sencha.com/forum/showthread.php?260106-Tooltips-on-forms-and-grid-are-not-resizing-to-the-size-of-the-text/page3#24
Specifically the following code at the beginning of the app:
delete Ext.tip.Tip.prototype.minWidth;