Sometimes besides just having a large quarantine as far as MB is concerned, sometimes you also end up with a large quarantine as far as the number of files is concerned. I found this on a computer where the user was complaining of slow speeds. In this case, they were repeatedly visiting a website that was infected with malware and the quarantine grew huge as a result. 57,996 objects in the Quarantine folder! I simply deleted everything in this folder and all was well again. I'm not sure if the slow speeds were a result of the Symantec's handling of the large quarantine or because it kept finding this stuff on the system.
As a side tip, on machines that have been running for a while and may be filled with junk, CCleaner is a free utility that works quite well to clean up such machines. I ran CCleaner against the machine and also against the primary user's profile and then defragged the hard drive. The machine was much more responsive after this work.