Take a look at the Arkel Tailrider. I'm using one on my MTB for commuting and so far it's working out quite nicely. I can stuff my work clothes, lunch, and a few other essentials inside. Definitely not your typical bulky pannier. My back was sweating profusely when using a backpack.
http://www.panniers.com/panniers/tai...asp?fl=1&site=
FWIW, you can always buy a Camelbak and remove the hydration pack to get some extra space inside. A change of clothes definitely fits into a HAWG (I have one of them too).