This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
As far as I can tell, the <jetty> task runs Jetty in the same JVM as Ant. If that is what you want, then you will have to increase the heap size used by Ant. If, however, you want to run Jetty in a separate JVM, use the <exec> or <java> tasks (make sure you set the fork option with the java task).