Exception in thread "MemoryMonitor" java.lang.OutOfMemoryError: Java heap space

I'm getting following error while runing SOAPUI pro.2.5.1... any clue what making tool to throw this error and how can I get rid of it ? tool is getting hung with this error.. experts please advice..

Exception in thread "MemoryMonitor" java.lang.OutOfMemoryError: Java heap space
        at com.eviware.soapui.support.monitor.MonitorPanel$Surface.run(MonitorPa
nel.java:211)
        at java.lang.Thread.run(Unknown Source)
Exception in thread "D3D Screen Updater" 15:33:13,135 ERROR [SoapUI] An error oc
cured [Java heap space], see error log for details
java.lang.OutOfMemoryError: Java heap space
        at java.lang.reflect.Array.newArray(Native Method)
Exception in thread "SoapUIGroovyClassLoader" java.lang.OutOfMemoryError: Java h
eap space
        at java.util.Arrays.copyOf(Unknown Source)
        at java.lang.AbstractStringBuilder.expandCapacity(Unknown Source)
        at java.lang.AbstractStringBuilder.append(Unknown Source)
        at java.lang.StringBuilder.append(Unknown Source)
        at java.lang.Throwable.toString(Throwable.java:344)
        at com.eviware.soapui.SoapUI.logError(SoapUI.java:735)
        at com.eviware.soapui.support.scripting.groovy.SoapUIGroovyClassLoader.r
un(SourceFile:75)
        at java.lang.Thread.run(Unknown Source)
Exception in thread "AWT-EventQueue-0" java.lang.OutOfMemoryError: Java heap spa
ce