Hi Ruben,
The problem is not with your java system. The problem is with your SUM tool.
In order to resolve the issue follow the below steps
1. Stop The SUMGui .
2. Stop the SL Controller (StartUP Command )
3. Open Startup Commandpresent in SUM root directory in edit mode .
4. Increase the Heap size and change '-Xmx512m' to '-Xmx768m' .If it does not work with the value 768 then keep on increasing the value.
5. Then please restart the Startup command (SL Controller ) and SAPGui .
Also do follow the sapnote
1907699 - java.lang.OutOfMemoryErrorerror in UPGRADE/EVALUATION/UPGEVAL
With Regards
Ashutosh Chaturvedi