The heap of the MQMonitor process should be increased, to a minimum 1.5GB.
This is configured in startMQMonitor.sh
Look for the section where minimum and maximum heap are configured
-Xms64m -Xmx512m
and increase the maximum
-Xms64m -Xmx1536m
Make sure you are using a 64-bit JRE if you want to allocate larger amounts of heap.