Realplayer make use of a memory cache (Clip Cache) to store the video you frequently watch (quicker access).
By default on 4MB is used, but of course you can increase the cache size according to your preferences
Click on the Realplayer button >Preferences > General > Content
In the "Clip Cache section" click on the "Settings" button
This shall open a new window, simply move the slider to increase/decrease the resources allocated to the cache