Hi John,
This is a topic that I think needs more stats before it can be answered properly. The Google guys, like on the link Glen points to, say that GWT almost always beats hand written scripts etc, but figures are hard to come across.
As Glen mentions, there is a danger that given the ease of development there is a risk of throwing everything GWT has into your application so code will bloat, not due to GWT but just to using toys that may not have been used if it was more difficult to write.
GWTs approach aims to reduce size of output code as much as possible - each permutation only contains code for a particlar browser; the compiler aggressively removes unused
Java code, unused i18n constants/messages are removed etc.
This article on this blog,
http://timepedia.blogspot.com/2007/06/gwt-demystified.html, gives a good idea on what the compiler does and actually lists figures of source code versus compiler code sizes.
//Adam