You can deactivate some portlets that you don't use from ext, it should be available in liferay's administration guide or developer guide on https://www.liferay.com/documentation. Depending on the amount of work you're prepared to put into this you might also want to think about dimensions of your caches and other settings. However, I consider an additional piece of memory to be a lot cheaper than investing the time (again on each update) to trim down Liferay, if it's only for memory reasons.
However, if you "easily" trim Liferay through some means, please provide your findings in the Liferay forums as well - it might be worth feeding it back.