I can confirm that increasing the heap with -Xmx does not allow the user to allocate more memory. I did confirm that setting the maximum heap to a smaller value causes a different error
Code:
Unhandled exception: java.lang.OutOfMemoryError: Java heap space at [org.usfirst.frc.team8330.robot.Robot.disabledPeriodic(Robot.java:45), edu.wpi.first.wpilibj.IterativeRobot.startCompetition(IterativeRobot.java:102), edu.wpi.first.wpilibj.RobotBase.main(RobotBase.java:234)]