You can try the following to see if the change will apply. Go to your windows explorer -> This PC -> OSDisk(C:) -> Progam Files -> dynaTrace -> the Dynatrace Client version you are running -> dtclient.ini.template.
In that file, you should see a line that looks similar to (-Xmx1536M) this where you can manually set the memory heap size for your client. Change it here and see if the change will stick. I am pretty sure you will have to close and reopen the client for the change to show up.