I installed Jenkins on my vserver. When I had a look at htop Jenkins was running with 30 thread, each was allowed to allocate 247MB memory and up to 1181MB virtual memory.>
You can also cut down the number of executors you have in your node. This may or may not help - it may be that Jenkins is smart enough to kill the thread when it isn't using an executor. Still, some more information would be useful: How many jobs do you have? What plugins are installed? With more details I could give better advice.