I'm not sure how load applet measures memory use, but generally if you close a program, all its executable code and libraries are kept in ram memory. You might notice this if you time how long it takes to start media player the first time, close it, and start again a second time right after. Might start faster the second time. Linux philosophy is free ram = wasted ram. After that, it's all about trying to be clever in deciding what to discard from ram when the need to load more stuff into it comes. Ideally you'd be able to predict the future and remove things that wont be used for awhile and load stuff that will be used soon, but predicting the future is notoriously difficult