If you are experiencing out of memory errors, it indicates that, for the way that you are using Mango, your Java VM needs to be configured to use more available memory. (Java by default will not consume unlimited amounts of RAM; startup options need to be set to determine the maximum memory it will use. The usual default max memory is 64M).
Depending upon how you start Mango, you need to add a Java option to set the maximum heap size. For example, if you use the Tomcat startup scripts you can set the JAVA_OPTS variable. See your startup script for details.
The option to add is -Xmx followed by the desired memory amount. For example, to set the max memory to 500M use the following:
General information on JVM tuning can be found here: http://edocs.bea.com/wls/docs61/perform/JVMTuning.html