IPhone – How to clear the Diagnostics & Usage data from the iPhone

I know this doesn't answer the question specifically, but are you sure you are on wi-fi when its available and not just solely on 3G? I've made that switch in the past and forgotten to go back. Also, have you installed any apps recently around the time the spike occrured?

If the application doesn't provide a way to delete documents, uninstalling and reinstalling the app is definitively the best option here.