Use projects archiving to reduce the amount of managed projects, thus the current file size, by removing the unnecessary ones (e.g.: closed projects) and saving them in separate files. When needed they can be restored and inserted back in the working file.
From Project menu, Archive project... , the current project will be saved as a RationalPlan Single Project (.xrp) file.
From Project menu, Restore project... , a previously archived project will be inserted back in the Project Portfolio.