It seems like your current approach is correct - when running jetty through maven, jetty is a thread inside the maven process. So increasing maven's heap will increase jetty's heap.
How are you setting MAVEN_OPTS?
One example I found looks like this:
MAVEN_OPTS='-Xmx256m -Xms10m' mvn clean jetty:run
MAVEN_OPTS is an environment variable here, and not passed to the JVM (who wouldn't know what to do with it).